feat: print includes answer only after unlocking solutions

- If solutions already unlocked (full mode), print button shows '打印题目+答案'
- If not unlocked, print button shows '打印题目' (problem only)
- No longer forces unlock when printing

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

SeedCppBasic_20_items.array.cc

@@ -0,0 +1,164 @@
+const CourseItem items[] = {
+  {
+    "cpp-basic-01-hello",
+    "C++基础01：环境配置与Hello World（VSCode）",
+    1,
+    "course:cpp-basic:01",
+    "# C++基础01：环境配置与Hello World（VSCode）\n\n## 学习目标\n- 安装并打开 VSCode，创建并运行第一个 C++14 程序\n- 学会新建文件、保存、运行、查看输出\n- 了解 `main()`、`#include <iostream>`、`cout`\n\n## 推荐视频（观看后写笔记）\n- 保姆级：VSCode + mingw64 配置 C/C++（BV1tg411N7Fq）\n  - https://www.bilibili.com/video/BV1tg411N7Fq/\n- 每天五分钟学C++：01 开发工具（BV1dK4y137bk，系列入口）\n  - https://www.bilibili.com/video/BV1dK4y137bk/\n\n## 参考图文\n- LoongBa 极简配置：GCC/VSCode/HelloWorld\n  - https://github.com/LoongBa/Cpp_Beginner_Guide\n\n## 练习（完成至少 2 题）\n- B2002 Hello, World! https://www.luogu.com.cn/problem/B2002\n- P1000 超级玛丽游戏 https://www.luogu.com.cn/problem/P1000\n\n## 提交要求\n- 在本题页面下方“学习笔记”区域写下：\n  1) 你安装了什么、遇到什么坑、怎么解决\n  2) 你的 HelloWorld 代码\n  3) 你学到的 3 个关键词\n",
+    {"cpp-basic","vscode","io","", "", ""}
+  },
+  {
+    "cpp-basic-02-io",
+    "C++基础02：输入输出与变量",
+    1,
+    "course:cpp-basic:02",
+    "# C++基础02：输入输出与变量\n\n## 学习目标\n- 会用 `cin` 读入、`cout` 输出\n- 理解变量：`int / long long / double / char / string`\n\n## 推荐视频\n- 每天五分钟学C++：02 输出、03 变量（系列入口见上）\n  - https://www.bilibili.com/video/BV1dK4y137bk/\n\n## 练习\n- P1001 A+B Problem https://www.luogu.com.cn/problem/P1001\n- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008\n- P5704 字母转换 https://www.luogu.com.cn/problem/P5704\n\n## 提交要求\n- 上传/填写学习笔记：写出 `cin/cout` 模板、常见错误（空格/换行）\n",
+    {"cpp-basic","io","types","", "", ""}
+  },
+  {
+    "cpp-basic-03-branch",
+    "C++基础03：分支结构（if / switch）",
+    2,
+    "course:cpp-basic:03",
+    "# C++基础03：分支结构（if / switch）\n\n## 学习目标\n- 会写 `if / else if / else` 与基本逻辑运算\n- 能处理边界与分类讨论\n\n## 练习\n- B2035 判断数正负 https://www.luogu.com.cn/problem/B2035\n- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711\n- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909\n\n## 提交要求\n- 笔记里写清：你如何找“边界”（例如 0、最小/最大、等于条件）\n",
+    {"cpp-basic","branch","logic","", "", ""}
+  },
+  {
+    "cpp-basic-04-loop",
+    "C++基础04：循环结构（for / while）",
+    2,
+    "course:cpp-basic:04",
+    "# C++基础04：循环结构（for / while）\n\n## 学习目标\n- 会用循环做：计数、累加、打印图形\n\n## 练习\n- B2083 画矩形 https://www.luogu.com.cn/problem/B2083\n- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421\n\n## 提交要求\n- 笔记里写：循环三要素（初始化/条件/更新）+ 你调试的方法\n",
+    {"cpp-basic","loop","debug","", "", ""}
+  },
+  {
+    "cpp-basic-05-array",
+    "C++基础05：数组入门（一维）",
+    3,
+    "course:cpp-basic:05",
+    "# C++基础05：数组入门（一维）\n\n## 学习目标\n- 会定义数组、遍历、统计\n\n## 练习\n- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427\n- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428\n\n## 提交要求\n- 笔记里写：数组下标从 0/1 的选择；如何避免越界\n",
+    {"cpp-basic","array","", "", "", ""}
+  },
+
+  // 06~20：新增课程
+  {
+    "cpp-basic-06-char-string",
+    "C++基础06：字符与字符串（char / string）",
+    3,
+    "course:cpp-basic:06",
+    "# C++基础06：字符与字符串（char / string）\n\n## 学习目标\n- 区分 `char`（一个字符）与 `string`（一串字符）\n- 会做：大小写转换、统计字母、简单加密\n\n## 重点\n- `char c='A'`，`string s=\"abc\"`\n- `s.size()` / `s[i]` / 遍历字符串\n\n## 练习（完成至少 2 题）\n- P5704 字母转换 https://www.luogu.com.cn/problem/P5704\n- P5733 自动修正 https://www.luogu.com.cn/problem/P5733\n- P1914 小书童——凯撒密码 https://www.luogu.com.cn/problem/P1914\n\n## 提交要求\n- 笔记写：`string` 下标访问与越界风险（最后一个下标是 size()-1）\n",
+    {"cpp-basic","string","char","", "", ""}
+  },
+  {
+    "cpp-basic-07-operator-cast",
+    "C++基础07：运算、取整与类型转换（/ %）",
+    3,
+    "course:cpp-basic:07",
+    "# C++基础07：运算、取整与类型转换（/ %）\n\n## 学习目标\n- 写对 `+ - * / %`\n- 理解：整数除法会向下取整；`%` 只能用于整数\n- 学会用 `long long` 防止溢出\n\n## 练习\n- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008\n- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421\n- P5708 三角形面积 https://www.luogu.com.cn/problem/P5708\n\n## 提交要求\n- 笔记写：什么时候要用 `long long`（例如钱、人数、乘法很大）\n",
+    {"cpp-basic","math","cast","", "", ""}
+  },
+  {
+    "cpp-basic-08-logic",
+    "C++基础08：逻辑运算与边界（&& || !）",
+    3,
+    "course:cpp-basic:08",
+    "# C++基础08：逻辑运算与边界（&& || !）\n\n## 学习目标\n- 能把题目里的“并且/或者/不是”写成条件\n- 学会找边界：等于、最小/最大、0\n\n## 练习\n- P5710 数的性质 https://www.luogu.com.cn/problem/P5710\n- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711\n- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909\n\n## 提交要求\n- 笔记写：你列出的边界测试用例（至少 3 个）\n",
+    {"cpp-basic","logic","boundary","", "", ""}
+  },
+  {
+    "cpp-basic-09-switch",
+    "C++基础09：switch 与枚举列情况",
+    3,
+    "course:cpp-basic:09",
+    "# C++基础09：switch 与枚举列情况\n\n## 学习目标\n- 会写 `switch/case/break`\n- 面对“情况很少”的题，会列举所有情况\n\n## 练习\n- P2433 小学数学N合一 https://www.luogu.com.cn/problem/P2433\n\n## 提交要求\n- 笔记写：为什么每个 case 后要 `break`（避免贯穿执行）\n",
+    {"cpp-basic","switch","enum","", "", ""}
+  },
+  {
+    "cpp-basic-10-for",
+    "C++基础10：for循环（计数与累加）",
+    3,
+    "course:cpp-basic:10",
+    "# C++基础10：for循环（计数与累加）\n\n## 学习目标\n- 熟练掌握 `for(初始化; 条件; 更新)`\n- 会写：求和、求最大最小、统计个数\n\n## 练习\n- P1425 小鱼的游泳时间 https://www.luogu.com.cn/problem/P1425\n- P5718 找最小值 https://www.luogu.com.cn/problem/P5718\n\n## 提交要求\n- 笔记写：循环“三要素”分别是什么\n",
+    {"cpp-basic","loop","for","", "", ""}
+  },
+  {
+    "cpp-basic-11-while",
+    "C++基础11：while循环（未知次数）",
+    3,
+    "course:cpp-basic:11",
+    "# C++基础11：while循环（未知次数）\n\n## 学习目标\n- 会用 `while` 处理：读到结束、读到 0 停止\n- 避免死循环：每次循环必须改变状态\n\n## 练习\n- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427\n- P1047 校门外的树（选做）https://www.luogu.com.cn/problem/P1047\n\n## 提交要求\n- 笔记写：你是如何避免死循环的（更新变量写在哪里）\n",
+    {"cpp-basic","loop","while","", "", ""}
+  },
+  {
+    "cpp-basic-12-nested",
+    "C++基础12：嵌套循环与打印图形",
+    3,
+    "course:cpp-basic:12",
+    "# C++基础12：嵌套循环与打印图形\n\n## 学习目标\n- 外层循环=行，内层循环=列\n- 能打印：矩形、三角形、乘法表\n\n## 练习\n- B2083 画矩形 https://www.luogu.com.cn/problem/B2083\n- P5717 三角形分类（练分类讨论 + 输出格式）https://www.luogu.com.cn/problem/P5717\n\n## 提交要求\n- 笔记写：你画图时是怎么想“行/列”的\n",
+    {"cpp-basic","nested-loop","print","", "", ""}
+  },
+  {
+    "cpp-basic-13-array-ops",
+    "C++基础13：数组进阶（统计/查找/反转）",
+    4,
+    "course:cpp-basic:13",
+    "# C++基础13：数组进阶（统计/查找/反转）\n\n## 学习目标\n- 用数组做：统计、查找位置、反转、去重（了解）\n\n## 练习\n- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428\n- P5727 冰雹猜想 https://www.luogu.com.cn/problem/P5727\n\n## 提交要求\n- 笔记写：数组越界的 2 个典型原因 + 你的检查方法\n",
+    {"cpp-basic","array","practice","", "", ""}
+  },
+  {
+    "cpp-basic-14-function",
+    "C++基础14：函数入门（把重复代码变成工具）",
+    4,
+    "course:cpp-basic:14",
+    "# C++基础14：函数入门（参数/返回值/作用域）\n\n## 学习目标\n- 会定义并调用函数\n- 知道局部变量作用域（{} 内有效）\n\n## 练习\n- P5735 距离函数 https://www.luogu.com.cn/problem/P5735\n- P5739 计算阶乘（也可用递归）https://www.luogu.com.cn/problem/P5739\n\n## 提交要求\n- 笔记写：你把哪段重复代码抽成了函数？为什么这样更清晰？\n",
+    {"cpp-basic","function","scope","", "", ""}
+  },
+  {
+    "cpp-basic-15-recursion",
+    "C++基础15：递归入门（选学）",
+    4,
+    "course:cpp-basic:15",
+    "# C++基础15：递归入门（选学）\n\n## 学习目标\n- 理解递归：函数调用自己\n- 必须有终止条件（否则会无限调用）\n\n## 练习\n- P5739 计算阶乘 https://www.luogu.com.cn/problem/P5739\n- P1028 数的计算（选做）https://www.luogu.com.cn/problem/P1028\n\n## 提交要求\n- 笔记写：你的递归“终止条件”是什么？\n",
+    {"cpp-basic","recursion","", "", "", ""}
+  },
+  {
+    "cpp-basic-16-struct",
+    "C++基础16：结构体 struct（选学）",
+    4,
+    "course:cpp-basic:16",
+    "# C++基础16：结构体 struct（选学）\n\n## 学习目标\n- 用 `struct` 把多个字段打包成一个“人物卡/记录”\n- 学会 `struct` 数组\n\n## 练习\n- P5740 最厉害的学生 https://www.luogu.com.cn/problem/P5740\n- P5744 培训 https://www.luogu.com.cn/problem/P5744\n\n## 提交要求\n- 笔记写：struct 适合用在什么场景（多个属性属于同一个对象）\n",
+    {"cpp-basic","struct","", "", "", ""}
+  },
+  {
+    "cpp-basic-17-sort",
+    "C++基础17：排序初步（sort）",
+    4,
+    "course:cpp-basic:17",
+    "# C++基础17：排序初步（sort）\n\n## 学习目标\n- 会用 `#include <algorithm>` 和 `sort`\n- 排序后做统计/查找会更简单\n\n## 练习\n- P1059 明明的随机数 https://www.luogu.com.cn/problem/P1059\n- P1093 奖学金（选做）https://www.luogu.com.cn/problem/P1093\n\n## 提交要求\n- 笔记写：sort 的区间是 `[begin, end)`（end 不包含）\n",
+    {"cpp-basic","sort","algorithm","", "", ""}
+  },
+  {
+    "cpp-basic-18-binary-search",
+    "C++基础18：二分查找初步（lower_bound）",
+    4,
+    "course:cpp-basic:18",
+    "# C++基础18：二分查找初步（lower_bound）\n\n## 学习目标\n- 理解“在有序数组里快速找答案”\n- 会用 `lower_bound` 找第一个 >= x 的位置\n\n## 练习\n- P2249 查找 https://www.luogu.com.cn/problem/P2249\n\n## 提交要求\n- 笔记写：二分查找的前提：数组必须有序\n",
+    {"cpp-basic","binary-search","", "", "", ""}
+  },
+  {
+    "cpp-basic-19-prefix-sum",
+    "C++基础19：前缀和（区间求和神器）",
+    4,
+    "course:cpp-basic:19",
+    "# C++基础19：前缀和（区间求和神器）\n\n## 学习目标\n- 会构造前缀和 `s[i]=s[i-1]+a[i]`\n- 会算区间和 `sum(l,r)=s[r]-s[l-1]`\n\n## 练习\n- P8218 求区间和（前缀和基础）https://www.luogu.com.cn/problem/P8218\n\n## 提交要求\n- 笔记写：下标偏移是怎么处理的（从 1 开始更方便）\n",
+    {"cpp-basic","prefix-sum","", "", "", ""}
+  },
+  {
+    "cpp-basic-20-simulation",
+    "C++基础20：模拟题与综合复盘",
+    4,
+    "course:cpp-basic:20",
+    "# C++基础20：模拟题与综合复盘\n\n## 学习目标\n- 学会按题意一步步做（模拟）\n- 复盘：总结常见错误 + 常用模板\n\n## 练习（从这里选 3~5 题做小测）\n- P1957 口算练习题 https://www.luogu.com.cn/problem/P1957\n- P1055 ISBN号码 https://www.luogu.com.cn/problem/P1055\n- P2433 小学数学N合一（回顾）https://www.luogu.com.cn/problem/P2433\n\n## 提交要求\n- 笔记写：\n  1) 你最常犯的 3 类错误\n  2) 你最常用的 5 行代码模板\n  3) 下周你准备怎么练\n",
+    {"cpp-basic","simulation","review","", "", ""}
+  },
+};

backend/src/db/sqlite_db.cc

@@ -754,125 +754,170 @@ void SeedDemoData(SqliteDb& db) {
         const char* tags[6];
       };
       const CourseItem items[] = {
-        {
+  {
-          "cpp-basic-01-hello",
+    "cpp-basic-01-hello",
-          "C++基础01：环境配置与Hello World（VSCode）",
+    "C++基础01：环境配置与Hello World（VSCode）",
-          1,
+    1,
-          "course:cpp-basic:01",
+    "course:cpp-basic:01",
-          R"MD(# C++基础01：环境配置与Hello World（VSCode）
+    "# C++基础01：环境配置与Hello World（VSCode）\n\n## 学习目标\n- 安装并打开 VSCode，创建并运行第一个 C++14 程序\n- 学会新建文件、保存、运行、查看输出\n- 了解 `main()`、`#include <iostream>`、`cout`\n\n## 推荐视频（观看后写笔记）\n- 保姆级：VSCode + mingw64 配置 C/C++（BV1tg411N7Fq）\n  - https://www.bilibili.com/video/BV1tg411N7Fq/\n- 每天五分钟学C++：01 开发工具（BV1dK4y137bk，系列入口）\n  - https://www.bilibili.com/video/BV1dK4y137bk/\n\n## 参考图文\n- LoongBa 极简配置：GCC/VSCode/HelloWorld\n  - https://github.com/LoongBa/Cpp_Beginner_Guide\n\n## 练习（完成至少 2 题）\n- B2002 Hello, World! https://www.luogu.com.cn/problem/B2002\n- P1000 超级玛丽游戏 https://www.luogu.com.cn/problem/P1000\n\n## 提交要求\n- 在本题页面下方“学习笔记”区域写下：\n  1) 你安装了什么、遇到什么坑、怎么解决\n  2) 你的 HelloWorld 代码\n  3) 你学到的 3 个关键词\n",
+    {"cpp-basic","vscode","io","", "", ""}
+  },
+  {
+    "cpp-basic-02-io",
+    "C++基础02：输入输出与变量",
+    1,
+    "course:cpp-basic:02",
+    "# C++基础02：输入输出与变量\n\n## 学习目标\n- 会用 `cin` 读入、`cout` 输出\n- 理解变量：`int / long long / double / char / string`\n\n## 推荐视频\n- 每天五分钟学C++：02 输出、03 变量（系列入口见上）\n  - https://www.bilibili.com/video/BV1dK4y137bk/\n\n## 练习\n- P1001 A+B Problem https://www.luogu.com.cn/problem/P1001\n- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008\n- P5704 字母转换 https://www.luogu.com.cn/problem/P5704\n\n## 提交要求\n- 上传/填写学习笔记：写出 `cin/cout` 模板、常见错误（空格/换行）\n",
+    {"cpp-basic","io","types","", "", ""}
+  },
+  {
+    "cpp-basic-03-branch",
+    "C++基础03：分支结构（if / switch）",
+    2,
+    "course:cpp-basic:03",
+    "# C++基础03：分支结构（if / switch）\n\n## 学习目标\n- 会写 `if / else if / else` 与基本逻辑运算\n- 能处理边界与分类讨论\n\n## 练习\n- B2035 判断数正负 https://www.luogu.com.cn/problem/B2035\n- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711\n- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909\n\n## 提交要求\n- 笔记里写清：你如何找“边界”（例如 0、最小/最大、等于条件）\n",
+    {"cpp-basic","branch","logic","", "", ""}
+  },
+  {
+    "cpp-basic-04-loop",
+    "C++基础04：循环结构（for / while）",
+    2,
+    "course:cpp-basic:04",
+    "# C++基础04：循环结构（for / while）\n\n## 学习目标\n- 会用循环做：计数、累加、打印图形\n\n## 练习\n- B2083 画矩形 https://www.luogu.com.cn/problem/B2083\n- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421\n\n## 提交要求\n- 笔记里写：循环三要素（初始化/条件/更新）+ 你调试的方法\n",
+    {"cpp-basic","loop","debug","", "", ""}
+  },
+  {
+    "cpp-basic-05-array",
+    "C++基础05：数组入门（一维）",
+    3,
+    "course:cpp-basic:05",
+    "# C++基础05：数组入门（一维）\n\n## 学习目标\n- 会定义数组、遍历、统计\n\n## 练习\n- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427\n- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428\n\n## 提交要求\n- 笔记里写：数组下标从 0/1 的选择；如何避免越界\n",
+    {"cpp-basic","array","", "", "", ""}
+  },
 
-## 学习目标
+  // 06~20：新增课程
-- 安装并打开 VSCode，创建并运行第一个 C++14 程序
+  {
-- 学会新建文件、保存、运行、查看输出
+    "cpp-basic-06-char-string",
-- 了解 `main()`、`#include <iostream>`、`cout`
+    "C++基础06：字符与字符串（char / string）",
+    3,
+    "course:cpp-basic:06",
+    "# C++基础06：字符与字符串（char / string）\n\n## 学习目标\n- 区分 `char`（一个字符）与 `string`（一串字符）\n- 会做：大小写转换、统计字母、简单加密\n\n## 重点\n- `char c='A'`，`string s=\"abc\"`\n- `s.size()` / `s[i]` / 遍历字符串\n\n## 练习（完成至少 2 题）\n- P5704 字母转换 https://www.luogu.com.cn/problem/P5704\n- P5733 自动修正 https://www.luogu.com.cn/problem/P5733\n- P1914 小书童——凯撒密码 https://www.luogu.com.cn/problem/P1914\n\n## 提交要求\n- 笔记写：`string` 下标访问与越界风险（最后一个下标是 size()-1）\n",
+    {"cpp-basic","string","char","", "", ""}
+  },
+  {
+    "cpp-basic-07-operator-cast",
+    "C++基础07：运算、取整与类型转换（/ %）",
+    3,
+    "course:cpp-basic:07",
+    "# C++基础07：运算、取整与类型转换（/ %）\n\n## 学习目标\n- 写对 `+ - * / %`\n- 理解：整数除法会向下取整；`%` 只能用于整数\n- 学会用 `long long` 防止溢出\n\n## 练习\n- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008\n- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421\n- P5708 三角形面积 https://www.luogu.com.cn/problem/P5708\n\n## 提交要求\n- 笔记写：什么时候要用 `long long`（例如钱、人数、乘法很大）\n",
+    {"cpp-basic","math","cast","", "", ""}
+  },
+  {
+    "cpp-basic-08-logic",
+    "C++基础08：逻辑运算与边界（&& || !）",
+    3,
+    "course:cpp-basic:08",
+    "# C++基础08：逻辑运算与边界（&& || !）\n\n## 学习目标\n- 能把题目里的“并且/或者/不是”写成条件\n- 学会找边界：等于、最小/最大、0\n\n## 练习\n- P5710 数的性质 https://www.luogu.com.cn/problem/P5710\n- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711\n- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909\n\n## 提交要求\n- 笔记写：你列出的边界测试用例（至少 3 个）\n",
+    {"cpp-basic","logic","boundary","", "", ""}
+  },
+  {
+    "cpp-basic-09-switch",
+    "C++基础09：switch 与枚举列情况",
+    3,
+    "course:cpp-basic:09",
+    "# C++基础09：switch 与枚举列情况\n\n## 学习目标\n- 会写 `switch/case/break`\n- 面对“情况很少”的题，会列举所有情况\n\n## 练习\n- P2433 小学数学N合一 https://www.luogu.com.cn/problem/P2433\n\n## 提交要求\n- 笔记写：为什么每个 case 后要 `break`（避免贯穿执行）\n",
+    {"cpp-basic","switch","enum","", "", ""}
+  },
+  {
+    "cpp-basic-10-for",
+    "C++基础10：for循环（计数与累加）",
+    3,
+    "course:cpp-basic:10",
+    "# C++基础10：for循环（计数与累加）\n\n## 学习目标\n- 熟练掌握 `for(初始化; 条件; 更新)`\n- 会写：求和、求最大最小、统计个数\n\n## 练习\n- P1425 小鱼的游泳时间 https://www.luogu.com.cn/problem/P1425\n- P5718 找最小值 https://www.luogu.com.cn/problem/P5718\n\n## 提交要求\n- 笔记写：循环“三要素”分别是什么\n",
+    {"cpp-basic","loop","for","", "", ""}
+  },
+  {
+    "cpp-basic-11-while",
+    "C++基础11：while循环（未知次数）",
+    3,
+    "course:cpp-basic:11",
+    "# C++基础11：while循环（未知次数）\n\n## 学习目标\n- 会用 `while` 处理：读到结束、读到 0 停止\n- 避免死循环：每次循环必须改变状态\n\n## 练习\n- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427\n- P1047 校门外的树（选做）https://www.luogu.com.cn/problem/P1047\n\n## 提交要求\n- 笔记写：你是如何避免死循环的（更新变量写在哪里）\n",
+    {"cpp-basic","loop","while","", "", ""}
+  },
+  {
+    "cpp-basic-12-nested",
+    "C++基础12：嵌套循环与打印图形",
+    3,
+    "course:cpp-basic:12",
+    "# C++基础12：嵌套循环与打印图形\n\n## 学习目标\n- 外层循环=行，内层循环=列\n- 能打印：矩形、三角形、乘法表\n\n## 练习\n- B2083 画矩形 https://www.luogu.com.cn/problem/B2083\n- P5717 三角形分类（练分类讨论 + 输出格式）https://www.luogu.com.cn/problem/P5717\n\n## 提交要求\n- 笔记写：你画图时是怎么想“行/列”的\n",
+    {"cpp-basic","nested-loop","print","", "", ""}
+  },
+  {
+    "cpp-basic-13-array-ops",
+    "C++基础13：数组进阶（统计/查找/反转）",
+    4,
+    "course:cpp-basic:13",
+    "# C++基础13：数组进阶（统计/查找/反转）\n\n## 学习目标\n- 用数组做：统计、查找位置、反转、去重（了解）\n\n## 练习\n- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428\n- P5727 冰雹猜想 https://www.luogu.com.cn/problem/P5727\n\n## 提交要求\n- 笔记写：数组越界的 2 个典型原因 + 你的检查方法\n",
+    {"cpp-basic","array","practice","", "", ""}
+  },
+  {
+    "cpp-basic-14-function",
+    "C++基础14：函数入门（把重复代码变成工具）",
+    4,
+    "course:cpp-basic:14",
+    "# C++基础14：函数入门（参数/返回值/作用域）\n\n## 学习目标\n- 会定义并调用函数\n- 知道局部变量作用域（{} 内有效）\n\n## 练习\n- P5735 距离函数 https://www.luogu.com.cn/problem/P5735\n- P5739 计算阶乘（也可用递归）https://www.luogu.com.cn/problem/P5739\n\n## 提交要求\n- 笔记写：你把哪段重复代码抽成了函数？为什么这样更清晰？\n",
+    {"cpp-basic","function","scope","", "", ""}
+  },
+  {
+    "cpp-basic-15-recursion",
+    "C++基础15：递归入门（选学）",
+    4,
+    "course:cpp-basic:15",
+    "# C++基础15：递归入门（选学）\n\n## 学习目标\n- 理解递归：函数调用自己\n- 必须有终止条件（否则会无限调用）\n\n## 练习\n- P5739 计算阶乘 https://www.luogu.com.cn/problem/P5739\n- P1028 数的计算（选做）https://www.luogu.com.cn/problem/P1028\n\n## 提交要求\n- 笔记写：你的递归“终止条件”是什么？\n",
+    {"cpp-basic","recursion","", "", "", ""}
+  },
+  {
+    "cpp-basic-16-struct",
+    "C++基础16：结构体 struct（选学）",
+    4,
+    "course:cpp-basic:16",
+    "# C++基础16：结构体 struct（选学）\n\n## 学习目标\n- 用 `struct` 把多个字段打包成一个“人物卡/记录”\n- 学会 `struct` 数组\n\n## 练习\n- P5740 最厉害的学生 https://www.luogu.com.cn/problem/P5740\n- P5744 培训 https://www.luogu.com.cn/problem/P5744\n\n## 提交要求\n- 笔记写：struct 适合用在什么场景（多个属性属于同一个对象）\n",
+    {"cpp-basic","struct","", "", "", ""}
+  },
+  {
+    "cpp-basic-17-sort",
+    "C++基础17：排序初步（sort）",
+    4,
+    "course:cpp-basic:17",
+    "# C++基础17：排序初步（sort）\n\n## 学习目标\n- 会用 `#include <algorithm>` 和 `sort`\n- 排序后做统计/查找会更简单\n\n## 练习\n- P1059 明明的随机数 https://www.luogu.com.cn/problem/P1059\n- P1093 奖学金（选做）https://www.luogu.com.cn/problem/P1093\n\n## 提交要求\n- 笔记写：sort 的区间是 `[begin, end)`（end 不包含）\n",
+    {"cpp-basic","sort","algorithm","", "", ""}
+  },
+  {
+    "cpp-basic-18-binary-search",
+    "C++基础18：二分查找初步（lower_bound）",
+    4,
+    "course:cpp-basic:18",
+    "# C++基础18：二分查找初步（lower_bound）\n\n## 学习目标\n- 理解“在有序数组里快速找答案”\n- 会用 `lower_bound` 找第一个 >= x 的位置\n\n## 练习\n- P2249 查找 https://www.luogu.com.cn/problem/P2249\n\n## 提交要求\n- 笔记写：二分查找的前提：数组必须有序\n",
+    {"cpp-basic","binary-search","", "", "", ""}
+  },
+  {
+    "cpp-basic-19-prefix-sum",
+    "C++基础19：前缀和（区间求和神器）",
+    4,
+    "course:cpp-basic:19",
+    "# C++基础19：前缀和（区间求和神器）\n\n## 学习目标\n- 会构造前缀和 `s[i]=s[i-1]+a[i]`\n- 会算区间和 `sum(l,r)=s[r]-s[l-1]`\n\n## 练习\n- P8218 求区间和（前缀和基础）https://www.luogu.com.cn/problem/P8218\n\n## 提交要求\n- 笔记写：下标偏移是怎么处理的（从 1 开始更方便）\n",
+    {"cpp-basic","prefix-sum","", "", "", ""}
+  },
+  {
+    "cpp-basic-20-simulation",
+    "C++基础20：模拟题与综合复盘",
+    4,
+    "course:cpp-basic:20",
+    "# C++基础20：模拟题与综合复盘\n\n## 学习目标\n- 学会按题意一步步做（模拟）\n- 复盘：总结常见错误 + 常用模板\n\n## 练习（从这里选 3~5 题做小测）\n- P1957 口算练习题 https://www.luogu.com.cn/problem/P1957\n- P1055 ISBN号码 https://www.luogu.com.cn/problem/P1055\n- P2433 小学数学N合一（回顾）https://www.luogu.com.cn/problem/P2433\n\n## 提交要求\n- 笔记写：\n  1) 你最常犯的 3 类错误\n  2) 你最常用的 5 行代码模板\n  3) 下周你准备怎么练\n",
+    {"cpp-basic","simulation","review","", "", ""}
+  },
+};
 
-## 推荐视频（观看后写笔记）
-- 保姆级：VSCode + mingw64 配置 C/C++（BV1tg411N7Fq）
-  - https://www.bilibili.com/video/BV1tg411N7Fq/
-- 每天五分钟学C++：01 开发工具（BV1dK4y137bk，系列入口）
-  - https://www.bilibili.com/video/BV1dK4y137bk/
-
-## 参考图文
-- LoongBa 极简配置：GCC/VSCode/HelloWorld
-  - https://github.com/LoongBa/Cpp_Beginner_Guide
-
-## 练习（完成至少 2 题）
-- B2002 Hello, World! https://www.luogu.com.cn/problem/B2002
-- P1000 超级玛丽游戏 https://www.luogu.com.cn/problem/P1000
-
-## 提交要求
-- 在本题页面下方“学习笔记”区域写下：
-  1) 你安装了什么、遇到什么坑、怎么解决
-  2) 你的 HelloWorld 代码
-  3) 你学到的 3 个关键词
-)MD",
-          {"cpp-basic", "vscode", "io", "", "", ""}
-        },
-        {
-          "cpp-basic-02-io",
-          "C++基础02：输入输出与变量",
-          1,
-          "course:cpp-basic:02",
-          R"MD(# C++基础02：输入输出与变量
-
-## 学习目标
-- 会用 `cin` 读入、`cout` 输出
-- 理解变量：`int / long long / double / char / string`
-
-## 推荐视频
-- 每天五分钟学C++：02 输出、03 变量（系列入口见上）
-  - https://www.bilibili.com/video/BV1dK4y137bk/
-
-## 练习
-- P1001 A+B Problem https://www.luogu.com.cn/problem/P1001
-- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008
-- P5704 字母转换 https://www.luogu.com.cn/problem/P5704
-
-## 提交要求
-- 上传/填写学习笔记：写出 `cin/cout` 模板、常见错误（空格/换行）
-)MD",
-          {"cpp-basic", "io", "types", "", "", ""}
-        },
-        {
-          "cpp-basic-03-branch",
-          "C++基础03：分支结构（if / switch）",
-          2,
-          "course:cpp-basic:03",
-          R"MD(# C++基础03：分支结构（if / switch）
-
-## 学习目标
-- 会写 `if / else if / else` 与基本逻辑运算
-- 能处理边界与分类讨论
-
-## 练习
-- B2035 判断数正负 https://www.luogu.com.cn/problem/B2035
-- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711
-- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909
-
-## 提交要求
-- 笔记里写清：你如何找“边界”（例如 0、最小/最大、等于条件）
-)MD",
-          {"cpp-basic", "branch", "logic", "", "", ""}
-        },
-        {
-          "cpp-basic-04-loop",
-          "C++基础04：循环结构（for / while）",
-          2,
-          "course:cpp-basic:04",
-          R"MD(# C++基础04：循环结构（for / while）
-
-## 学习目标
-- 会用循环做：计数、累加、打印图形
-
-## 练习
-- B2083 画矩形 https://www.luogu.com.cn/problem/B2083
-- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421
-
-## 提交要求
-- 笔记里写：循环三要素（初始化/条件/更新）+ 你调试的方法
-)MD",
-          {"cpp-basic", "loop", "debug", "", "", ""}
-        },
-        {
-          "cpp-basic-05-array",
-          "C++基础05：数组入门（一维）",
-          3,
-          "course:cpp-basic:05",
-          R"MD(# C++基础05：数组入门（一维）
-
-## 学习目标
-- 会定义数组、遍历、统计
-
-## 练习
-- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427
-- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428
-
-## 提交要求
-- 笔记里写：数组下标从 0/1 的选择；如何避免越界
-)MD",
-          {"cpp-basic", "array", "", "", "", ""}
-        }
-      };
 
       for (const auto& it : items) {
         InsertProblem(raw, it.slug, it.title, it.md, it.diff, it.source, "", "", created);

backend/src/db/sqlite_db.cc.bak_cppbasic20

@@ -0,0 +1,892 @@
+#include "csp/db/sqlite_db.h"
+
+#include <chrono>
+#include <optional>
+#include <stdexcept>
+#include <string>
+#include <utility>
+
+namespace csp::db {
+
+namespace {
+
+void ThrowSqlite(int rc, sqlite3* db, const char* what) {
+  if (rc == SQLITE_OK || rc == SQLITE_DONE || rc == SQLITE_ROW) return;
+  const char* msg = db ? sqlite3_errmsg(db) : "";
+  throw std::runtime_error(std::string(what) + ": " + msg);
+}
+
+int64_t NowSec() {
+  using namespace std::chrono;
+  return duration_cast<seconds>(system_clock::now().time_since_epoch()).count();
+}
+
+bool ColumnExists(sqlite3* db, const char* table, const char* col) {
+  sqlite3_stmt* stmt = nullptr;
+  const std::string sql = std::string("PRAGMA table_info(") + table + ")";
+  const int rc = sqlite3_prepare_v2(db, sql.c_str(), -1, &stmt, nullptr);
+  if (rc != SQLITE_OK) {
+    if (stmt) sqlite3_finalize(stmt);
+    return false;
+  }
+
+  bool found = false;
+  while (sqlite3_step(stmt) == SQLITE_ROW) {
+    const unsigned char* name = sqlite3_column_text(stmt, 1);
+    if (name && std::string(reinterpret_cast<const char*>(name)) == col) {
+      found = true;
+      break;
+    }
+  }
+  sqlite3_finalize(stmt);
+  return found;
+}
+
+void EnsureColumn(SqliteDb& db,
+                 const char* table,
+                 const char* col_name,
+                 const char* col_def) {
+  if (ColumnExists(db.raw(), table, col_name)) return;
+  db.Exec(std::string("ALTER TABLE ") + table + " ADD COLUMN " + col_def + ";");
+}
+
+int CountRows(sqlite3* db, const char* table) {
+  sqlite3_stmt* stmt = nullptr;
+  const std::string sql = std::string("SELECT COUNT(1) FROM ") + table;
+  int rc = sqlite3_prepare_v2(db, sql.c_str(), -1, &stmt, nullptr);
+  if (rc != SQLITE_OK) {
+    if (stmt) sqlite3_finalize(stmt);
+    return 0;
+  }
+
+  rc = sqlite3_step(stmt);
+  int count = 0;
+  if (rc == SQLITE_ROW) count = sqlite3_column_int(stmt, 0);
+  sqlite3_finalize(stmt);
+  return count;
+}
+
+std::optional<int64_t> QueryOneId(sqlite3* db, const std::string& sql) {
+  sqlite3_stmt* stmt = nullptr;
+  const int rc = sqlite3_prepare_v2(db, sql.c_str(), -1, &stmt, nullptr);
+  if (rc != SQLITE_OK) {
+    if (stmt) sqlite3_finalize(stmt);
+    return std::nullopt;
+  }
+  if (sqlite3_step(stmt) != SQLITE_ROW) {
+    sqlite3_finalize(stmt);
+    return std::nullopt;
+  }
+  const auto id = sqlite3_column_int64(stmt, 0);
+  sqlite3_finalize(stmt);
+  return id;
+}
+
+void InsertProblem(sqlite3* db,
+                   const std::string& slug,
+                   const std::string& title,
+                   const std::string& statement,
+                   int difficulty,
+                   const std::string& source,
+                   const std::string& sample_in,
+                   const std::string& sample_out,
+                   int64_t created_at) {
+  sqlite3_stmt* stmt = nullptr;
+  const char* sql =
+      "INSERT INTO problems(slug,title,statement_md,difficulty,source,sample_input,sample_output,created_at) "
+      "VALUES(?,?,?,?,?,?,?,?)";
+  ThrowSqlite(sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr), db,
+              "prepare insert problem");
+  ThrowSqlite(sqlite3_bind_text(stmt, 1, slug.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind problem.slug");
+  ThrowSqlite(sqlite3_bind_text(stmt, 2, title.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind problem.title");
+  ThrowSqlite(sqlite3_bind_text(stmt, 3, statement.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind problem.statement");
+  ThrowSqlite(sqlite3_bind_int(stmt, 4, difficulty), db,
+              "bind problem.difficulty");
+  ThrowSqlite(sqlite3_bind_text(stmt, 5, source.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind problem.source");
+  ThrowSqlite(sqlite3_bind_text(stmt, 6, sample_in.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind problem.sample_input");
+  ThrowSqlite(sqlite3_bind_text(stmt, 7, sample_out.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind problem.sample_output");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 8, created_at), db,
+              "bind problem.created_at");
+  ThrowSqlite(sqlite3_step(stmt), db, "insert problem");
+  sqlite3_finalize(stmt);
+}
+
+void InsertProblemTag(sqlite3* db, int64_t problem_id, const std::string& tag) {
+  sqlite3_stmt* stmt = nullptr;
+  const char* sql =
+      "INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES(?,?)";
+  ThrowSqlite(sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr), db,
+              "prepare insert problem_tag");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 1, problem_id), db,
+              "bind problem_tag.problem_id");
+  ThrowSqlite(sqlite3_bind_text(stmt, 2, tag.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind problem_tag.tag");
+  ThrowSqlite(sqlite3_step(stmt), db, "insert problem_tag");
+  sqlite3_finalize(stmt);
+}
+
+void InsertKbArticle(sqlite3* db,
+                     const std::string& slug,
+                     const std::string& title,
+                     const std::string& content_md,
+                     int64_t created_at) {
+  sqlite3_stmt* stmt = nullptr;
+  const char* sql =
+      "INSERT INTO kb_articles(slug,title,content_md,created_at) VALUES(?,?,?,?)";
+  ThrowSqlite(sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr), db,
+              "prepare insert kb_article");
+  ThrowSqlite(sqlite3_bind_text(stmt, 1, slug.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind kb_article.slug");
+  ThrowSqlite(sqlite3_bind_text(stmt, 2, title.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind kb_article.title");
+  ThrowSqlite(sqlite3_bind_text(stmt, 3, content_md.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind kb_article.content");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 4, created_at), db,
+              "bind kb_article.created_at");
+  ThrowSqlite(sqlite3_step(stmt), db, "insert kb_article");
+  sqlite3_finalize(stmt);
+}
+
+void InsertKbLink(sqlite3* db, int64_t article_id, int64_t problem_id) {
+  sqlite3_stmt* stmt = nullptr;
+  const char* sql =
+      "INSERT OR IGNORE INTO kb_article_links(article_id,problem_id) VALUES(?,?)";
+  ThrowSqlite(sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr), db,
+              "prepare insert kb_article_link");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 1, article_id), db,
+              "bind kb_article_link.article_id");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 2, problem_id), db,
+              "bind kb_article_link.problem_id");
+  ThrowSqlite(sqlite3_step(stmt), db, "insert kb_article_link");
+  sqlite3_finalize(stmt);
+}
+
+void InsertContest(sqlite3* db,
+                   const std::string& title,
+                   int64_t starts_at,
+                   int64_t ends_at,
+                   const std::string& rule_json) {
+  sqlite3_stmt* stmt = nullptr;
+  const char* sql =
+      "INSERT INTO contests(title,starts_at,ends_at,rule_json) VALUES(?,?,?,?)";
+  ThrowSqlite(sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr), db,
+              "prepare insert contest");
+  ThrowSqlite(sqlite3_bind_text(stmt, 1, title.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind contest.title");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 2, starts_at), db,
+              "bind contest.starts_at");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 3, ends_at), db,
+              "bind contest.ends_at");
+  ThrowSqlite(sqlite3_bind_text(stmt, 4, rule_json.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind contest.rule_json");
+  ThrowSqlite(sqlite3_step(stmt), db, "insert contest");
+  sqlite3_finalize(stmt);
+}
+
+void InsertContestProblem(sqlite3* db,
+                          int64_t contest_id,
+                          int64_t problem_id,
+                          int idx) {
+  sqlite3_stmt* stmt = nullptr;
+  const char* sql =
+      "INSERT OR IGNORE INTO contest_problems(contest_id,problem_id,idx) VALUES(?,?,?)";
+  ThrowSqlite(sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr), db,
+              "prepare insert contest_problem");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 1, contest_id), db,
+              "bind contest_problem.contest_id");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 2, problem_id), db,
+              "bind contest_problem.problem_id");
+  ThrowSqlite(sqlite3_bind_int(stmt, 3, idx), db,
+              "bind contest_problem.idx");
+  ThrowSqlite(sqlite3_step(stmt), db, "insert contest_problem");
+  sqlite3_finalize(stmt);
+}
+
+void InsertRedeemItem(sqlite3* db,
+                      const std::string& name,
+                      const std::string& description,
+                      const std::string& unit_label,
+                      int holiday_cost,
+                      int studyday_cost,
+                      int is_active,
+                      int is_global,
+                      int64_t created_by,
+                      int64_t created_at) {
+  sqlite3_stmt* stmt = nullptr;
+  const char* sql =
+      "INSERT INTO redeem_items("
+      "name,description,unit_label,holiday_cost,studyday_cost,is_active,is_global,created_by,created_at,updated_at"
+      ") VALUES(?,?,?,?,?,?,?,?,?,?)";
+  ThrowSqlite(sqlite3_prepare_v2(db, sql, -1, &stmt, nullptr), db,
+              "prepare insert redeem_item");
+  ThrowSqlite(sqlite3_bind_text(stmt, 1, name.c_str(), -1, SQLITE_TRANSIENT), db,
+              "bind redeem_item.name");
+  ThrowSqlite(sqlite3_bind_text(stmt, 2, description.c_str(), -1, SQLITE_TRANSIENT),
+              db, "bind redeem_item.description");
+  ThrowSqlite(sqlite3_bind_text(stmt, 3, unit_label.c_str(), -1, SQLITE_TRANSIENT),
+              db, "bind redeem_item.unit_label");
+  ThrowSqlite(sqlite3_bind_int(stmt, 4, holiday_cost), db,
+              "bind redeem_item.holiday_cost");
+  ThrowSqlite(sqlite3_bind_int(stmt, 5, studyday_cost), db,
+              "bind redeem_item.studyday_cost");
+  ThrowSqlite(sqlite3_bind_int(stmt, 6, is_active), db,
+              "bind redeem_item.is_active");
+  ThrowSqlite(sqlite3_bind_int(stmt, 7, is_global), db,
+              "bind redeem_item.is_global");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 8, created_by), db,
+              "bind redeem_item.created_by");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 9, created_at), db,
+              "bind redeem_item.created_at");
+  ThrowSqlite(sqlite3_bind_int64(stmt, 10, created_at), db,
+              "bind redeem_item.updated_at");
+  ThrowSqlite(sqlite3_step(stmt), db, "insert redeem_item");
+  sqlite3_finalize(stmt);
+}
+
+}  // namespace
+
+SqliteDb SqliteDb::OpenFile(const std::string& path) {
+  sqlite3* db = nullptr;
+  const int rc = sqlite3_open(path.c_str(), &db);
+  if (rc != SQLITE_OK) {
+    const char* msg = db ? sqlite3_errmsg(db) : "";
+    if (db) sqlite3_close(db);
+    throw std::runtime_error(std::string("sqlite3_open failed: ") + msg);
+  }
+  return SqliteDb(db);
+}
+
+SqliteDb SqliteDb::OpenMemory() {
+  sqlite3* db = nullptr;
+  const int rc = sqlite3_open(":memory:", &db);
+  ThrowSqlite(rc, db, "sqlite3_open(:memory:) failed");
+  return SqliteDb(db);
+}
+
+SqliteDb::~SqliteDb() {
+  if (db_) sqlite3_close(db_);
+}
+
+SqliteDb::SqliteDb(SqliteDb&& other) noexcept : db_(other.db_) {
+  other.db_ = nullptr;
+}
+
+SqliteDb& SqliteDb::operator=(SqliteDb&& other) noexcept {
+  if (this == &other) return *this;
+  if (db_) sqlite3_close(db_);
+  db_ = other.db_;
+  other.db_ = nullptr;
+  return *this;
+}
+
+void SqliteDb::Exec(const std::string& sql) {
+  char* err = nullptr;
+  const int rc = sqlite3_exec(db_, sql.c_str(), nullptr, nullptr, &err);
+  if (rc != SQLITE_OK) {
+    std::string msg = err ? err : "";
+    sqlite3_free(err);
+    ThrowSqlite(rc, db_, msg.c_str());
+  }
+}
+
+void ApplyMigrations(SqliteDb& db) {
+  // Keep it simple for MVP: create missing tables, then patch missing columns.
+  db.Exec("PRAGMA foreign_keys = ON;");
+
+  db.Exec(R"SQL(
+CREATE TABLE IF NOT EXISTS users (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  username TEXT NOT NULL UNIQUE,
+  password_salt TEXT NOT NULL,
+  password_hash TEXT NOT NULL,
+  rating INTEGER NOT NULL DEFAULT 0,
+  created_at INTEGER NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS sessions (
+  token TEXT PRIMARY KEY,
+  user_id INTEGER NOT NULL,
+  expires_at INTEGER NOT NULL,
+  created_at INTEGER NOT NULL,
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS problems (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  slug TEXT NOT NULL UNIQUE,
+  title TEXT NOT NULL,
+  statement_md TEXT NOT NULL,
+  difficulty INTEGER NOT NULL DEFAULT 1,
+  source TEXT NOT NULL DEFAULT "",
+  statement_url TEXT NOT NULL DEFAULT "",
+  llm_profile_json TEXT NOT NULL DEFAULT "{}",
+  sample_input TEXT NOT NULL DEFAULT "",
+  sample_output TEXT NOT NULL DEFAULT "",
+  created_at INTEGER NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS problem_tags (
+  problem_id INTEGER NOT NULL,
+  tag TEXT NOT NULL,
+  PRIMARY KEY(problem_id, tag),
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS submissions (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id INTEGER NOT NULL,
+  problem_id INTEGER NOT NULL,
+  contest_id INTEGER,
+  language TEXT NOT NULL,
+  code TEXT NOT NULL,
+  status TEXT NOT NULL,
+  score INTEGER NOT NULL DEFAULT 0,
+  time_ms INTEGER NOT NULL DEFAULT 0,
+  memory_kb INTEGER NOT NULL DEFAULT 0,
+  compile_log TEXT NOT NULL DEFAULT "",
+  runtime_log TEXT NOT NULL DEFAULT "",
+  created_at INTEGER NOT NULL,
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE,
+  FOREIGN KEY(contest_id) REFERENCES contests(id) ON DELETE SET NULL
+);
+
+CREATE TABLE IF NOT EXISTS wrong_book (
+  user_id INTEGER NOT NULL,
+  problem_id INTEGER NOT NULL,
+  last_submission_id INTEGER,
+  note TEXT NOT NULL DEFAULT "",
+  note_score INTEGER NOT NULL DEFAULT 0,
+  note_rating INTEGER NOT NULL DEFAULT 0,
+  note_feedback_md TEXT NOT NULL DEFAULT "",
+  note_scored_at INTEGER NOT NULL DEFAULT 0,
+  updated_at INTEGER NOT NULL,
+  PRIMARY KEY(user_id, problem_id),
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE,
+  FOREIGN KEY(last_submission_id) REFERENCES submissions(id) ON DELETE SET NULL
+);
+
+CREATE TABLE IF NOT EXISTS contests (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  title TEXT NOT NULL,
+  starts_at INTEGER NOT NULL,
+  ends_at INTEGER NOT NULL,
+  rule_json TEXT NOT NULL DEFAULT "{}"
+);
+
+CREATE TABLE IF NOT EXISTS contest_problems (
+  contest_id INTEGER NOT NULL,
+  problem_id INTEGER NOT NULL,
+  idx INTEGER NOT NULL,
+  PRIMARY KEY(contest_id, problem_id),
+  FOREIGN KEY(contest_id) REFERENCES contests(id) ON DELETE CASCADE,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS contest_registrations (
+  contest_id INTEGER NOT NULL,
+  user_id INTEGER NOT NULL,
+  registered_at INTEGER NOT NULL,
+  PRIMARY KEY(contest_id, user_id),
+  FOREIGN KEY(contest_id) REFERENCES contests(id) ON DELETE CASCADE,
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS kb_articles (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  slug TEXT NOT NULL UNIQUE,
+  title TEXT NOT NULL,
+  content_md TEXT NOT NULL,
+  created_at INTEGER NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS kb_article_links (
+  article_id INTEGER NOT NULL,
+  problem_id INTEGER NOT NULL,
+  PRIMARY KEY(article_id, problem_id),
+  FOREIGN KEY(article_id) REFERENCES kb_articles(id) ON DELETE CASCADE,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS import_jobs (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  status TEXT NOT NULL,
+  trigger TEXT NOT NULL DEFAULT "manual",
+  total_count INTEGER NOT NULL DEFAULT 0,
+  processed_count INTEGER NOT NULL DEFAULT 0,
+  success_count INTEGER NOT NULL DEFAULT 0,
+  failed_count INTEGER NOT NULL DEFAULT 0,
+  options_json TEXT NOT NULL DEFAULT "{}",
+  last_error TEXT NOT NULL DEFAULT "",
+  started_at INTEGER NOT NULL,
+  finished_at INTEGER,
+  updated_at INTEGER NOT NULL,
+  created_at INTEGER NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS import_job_items (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  job_id INTEGER NOT NULL,
+  source_path TEXT NOT NULL,
+  status TEXT NOT NULL DEFAULT "queued",
+  title TEXT NOT NULL DEFAULT "",
+  difficulty INTEGER NOT NULL DEFAULT 0,
+  problem_id INTEGER,
+  error_text TEXT NOT NULL DEFAULT "",
+  started_at INTEGER,
+  finished_at INTEGER,
+  updated_at INTEGER NOT NULL,
+  created_at INTEGER NOT NULL,
+  FOREIGN KEY(job_id) REFERENCES import_jobs(id) ON DELETE CASCADE,
+  UNIQUE(job_id, source_path)
+);
+
+CREATE TABLE IF NOT EXISTS problem_drafts (
+  user_id INTEGER NOT NULL,
+  problem_id INTEGER NOT NULL,
+  language TEXT NOT NULL DEFAULT "cpp",
+  code TEXT NOT NULL DEFAULT "",
+  stdin TEXT NOT NULL DEFAULT "",
+  updated_at INTEGER NOT NULL,
+  created_at INTEGER NOT NULL,
+  PRIMARY KEY(user_id, problem_id),
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS problem_solution_jobs (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  problem_id INTEGER NOT NULL,
+  status TEXT NOT NULL DEFAULT "queued",
+  progress INTEGER NOT NULL DEFAULT 0,
+  message TEXT NOT NULL DEFAULT "",
+  created_by INTEGER NOT NULL DEFAULT 0,
+  max_solutions INTEGER NOT NULL DEFAULT 3,
+  created_at INTEGER NOT NULL,
+  started_at INTEGER,
+  finished_at INTEGER,
+  updated_at INTEGER NOT NULL,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS problem_solutions (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  problem_id INTEGER NOT NULL,
+  variant INTEGER NOT NULL DEFAULT 1,
+  title TEXT NOT NULL DEFAULT "",
+  idea_md TEXT NOT NULL DEFAULT "",
+  explanation_md TEXT NOT NULL DEFAULT "",
+  code_cpp TEXT NOT NULL DEFAULT "",
+  complexity TEXT NOT NULL DEFAULT "",
+  tags_json TEXT NOT NULL DEFAULT "[]",
+  source TEXT NOT NULL DEFAULT "llm",
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS problem_solution_view_logs (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id INTEGER NOT NULL,
+  problem_id INTEGER NOT NULL,
+  day_key TEXT NOT NULL,
+  viewed_at INTEGER NOT NULL,
+  charged INTEGER NOT NULL DEFAULT 0,
+  cost INTEGER NOT NULL DEFAULT 0,
+  created_at INTEGER NOT NULL,
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE,
+  FOREIGN KEY(problem_id) REFERENCES problems(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS submission_feedback (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  submission_id INTEGER NOT NULL UNIQUE,
+  feedback_md TEXT NOT NULL DEFAULT "",
+  links_json TEXT NOT NULL DEFAULT "[]",
+  model_name TEXT NOT NULL DEFAULT "",
+  status TEXT NOT NULL DEFAULT "ready",
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL,
+  FOREIGN KEY(submission_id) REFERENCES submissions(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS redeem_items (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  name TEXT NOT NULL,
+  description TEXT NOT NULL DEFAULT "",
+  unit_label TEXT NOT NULL DEFAULT "小时",
+  holiday_cost INTEGER NOT NULL DEFAULT 5,
+  studyday_cost INTEGER NOT NULL DEFAULT 25,
+  duration_minutes INTEGER NOT NULL DEFAULT 0,
+  is_active INTEGER NOT NULL DEFAULT 1,
+  is_global INTEGER NOT NULL DEFAULT 1,
+  created_by INTEGER NOT NULL DEFAULT 0,
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS redeem_records (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id INTEGER NOT NULL,
+  item_id INTEGER NOT NULL,
+  item_name TEXT NOT NULL,
+  quantity INTEGER NOT NULL DEFAULT 1,
+  day_type TEXT NOT NULL DEFAULT "studyday",
+  unit_cost INTEGER NOT NULL DEFAULT 0,
+  total_cost INTEGER NOT NULL DEFAULT 0,
+  note TEXT NOT NULL DEFAULT "",
+  created_at INTEGER NOT NULL,
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE,
+  FOREIGN KEY(item_id) REFERENCES redeem_items(id) ON DELETE RESTRICT
+);
+
+CREATE TABLE IF NOT EXISTS daily_task_logs (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id INTEGER NOT NULL,
+  task_code TEXT NOT NULL,
+  day_key TEXT NOT NULL,
+  reward INTEGER NOT NULL DEFAULT 1,
+  created_at INTEGER NOT NULL,
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE,
+  UNIQUE(user_id, task_code, day_key)
+);
+  )SQL");
+
+  // Backward-compatible schema upgrades for existing deployments.
+  EnsureColumn(db, "problems", "sample_input",
+               "sample_input TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problems", "sample_output",
+               "sample_output TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problems", "statement_url",
+               "statement_url TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problems", "llm_profile_json",
+               "llm_profile_json TEXT NOT NULL DEFAULT '{}'");
+  EnsureColumn(db, "import_jobs", "trigger",
+               "trigger TEXT NOT NULL DEFAULT 'manual'");
+  EnsureColumn(db, "import_jobs", "options_json",
+               "options_json TEXT NOT NULL DEFAULT '{}'");
+  EnsureColumn(db, "import_jobs", "last_error",
+               "last_error TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "submissions", "contest_id", "contest_id INTEGER");
+  EnsureColumn(db, "submissions", "compile_log",
+               "compile_log TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "submissions", "runtime_log",
+               "runtime_log TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problem_drafts", "stdin", "stdin TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problem_solution_jobs", "max_solutions",
+               "max_solutions INTEGER NOT NULL DEFAULT 3");
+  EnsureColumn(db, "wrong_book", "note_score", "note_score INTEGER NOT NULL DEFAULT 0");
+  EnsureColumn(db, "wrong_book", "note_rating", "note_rating INTEGER NOT NULL DEFAULT 0");
+  EnsureColumn(db, "wrong_book", "note_feedback_md", "note_feedback_md TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "wrong_book", "note_scored_at", "note_scored_at INTEGER NOT NULL DEFAULT 0");
+  EnsureColumn(db, "wrong_book", "note_images_json", "note_images_json TEXT NOT NULL DEFAULT '[]'");
+  EnsureColumn(db, "problem_solutions", "variant", "variant INTEGER NOT NULL DEFAULT 1");
+  EnsureColumn(db, "problem_solutions", "idea_md", "idea_md TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problem_solutions", "explanation_md",
+               "explanation_md TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problem_solutions", "code_cpp", "code_cpp TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problem_solutions", "complexity",
+               "complexity TEXT NOT NULL DEFAULT ''");
+  EnsureColumn(db, "problem_solutions", "tags_json", "tags_json TEXT NOT NULL DEFAULT '[]'");
+  EnsureColumn(db, "redeem_items", "duration_minutes",
+               "duration_minutes INTEGER NOT NULL DEFAULT 0");
+
+  // Build indexes after compatibility ALTERs so old schemas won't fail on
+  // missing columns (e.g. legacy submissions table without contest_id).
+  db.Exec(R"SQL(
+CREATE INDEX IF NOT EXISTS idx_submissions_user_created_at ON submissions(user_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_submissions_problem_created_at ON submissions(problem_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_submissions_contest_user_created_at ON submissions(contest_id, user_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_problem_tags_tag ON problem_tags(tag);
+CREATE INDEX IF NOT EXISTS idx_kb_article_links_problem_id ON kb_article_links(problem_id);
+CREATE INDEX IF NOT EXISTS idx_import_jobs_created_at ON import_jobs(created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_import_jobs_status ON import_jobs(status, updated_at DESC);
+CREATE INDEX IF NOT EXISTS idx_import_job_items_job_status ON import_job_items(job_id, status, updated_at DESC);
+CREATE INDEX IF NOT EXISTS idx_problem_drafts_updated ON problem_drafts(updated_at DESC);
+CREATE INDEX IF NOT EXISTS idx_problem_solution_jobs_problem ON problem_solution_jobs(problem_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_problem_solutions_problem ON problem_solutions(problem_id, variant, id);
+CREATE INDEX IF NOT EXISTS idx_solution_view_logs_user_problem ON problem_solution_view_logs(user_id, problem_id, viewed_at DESC);
+CREATE INDEX IF NOT EXISTS idx_solution_view_logs_user_day ON problem_solution_view_logs(user_id, day_key, viewed_at DESC);
+CREATE INDEX IF NOT EXISTS idx_redeem_items_active ON redeem_items(is_active, id);
+CREATE INDEX IF NOT EXISTS idx_redeem_records_user_created ON redeem_records(user_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_daily_task_logs_user_day ON daily_task_logs(user_id, day_key, created_at DESC);
+  )SQL");
+}
+
+void SeedDemoData(SqliteDb& db) {
+  sqlite3* raw = db.raw();
+  const int64_t now = NowSec();
+
+  if (CountRows(raw, "problems") == 0) {
+    InsertProblem(
+        raw,
+        "a-plus-b",
+        "A + B",
+        "给定两个整数 A 与 B，输出它们的和。",
+        1,
+        "CSP-入门",
+        "1 2\n",
+        "3\n",
+        now);
+
+    InsertProblem(
+        raw,
+        "fibonacci-n",
+        "Fibonacci 第 n 项",
+        "输入 n (0<=n<=40)，输出第 n 项 Fibonacci 数。",
+        2,
+        "CSP-基础",
+        "10\n",
+        "55\n",
+        now);
+
+    InsertProblem(
+        raw,
+        "sort-numbers",
+        "整数排序",
+        "输入 n 和 n 个整数，按升序输出。",
+        2,
+        "CSP-基础",
+        "5\n5 1 4 2 3\n",
+        "1 2 3 4 5\n",
+        now);
+  }
+
+  if (CountRows(raw, "problem_tags") == 0) {
+    const auto p1 =
+        QueryOneId(raw, "SELECT id FROM problems WHERE slug='a-plus-b'");
+    const auto p2 =
+        QueryOneId(raw, "SELECT id FROM problems WHERE slug='fibonacci-n'");
+    const auto p3 =
+        QueryOneId(raw, "SELECT id FROM problems WHERE slug='sort-numbers'");
+    if (p1) {
+      InsertProblemTag(raw, *p1, "math");
+      InsertProblemTag(raw, *p1, "implementation");
+    }
+    if (p2) {
+      InsertProblemTag(raw, *p2, "dp");
+      InsertProblemTag(raw, *p2, "recursion");
+    }
+    if (p3) {
+      InsertProblemTag(raw, *p3, "sort");
+      InsertProblemTag(raw, *p3, "array");
+    }
+  }
+
+  if (CountRows(raw, "kb_articles") == 0) {
+    InsertKbArticle(
+        raw,
+        "cpp-fast-io",
+        "C++ 快速输入输出",
+        "# C++ 快速输入输出\n\n在 OI/CSP 中，建议关闭同步并解绑 cin/cout：\n\n```cpp\nstd::ios::sync_with_stdio(false);\nstd::cin.tie(nullptr);\n```\n",
+        now);
+
+    InsertKbArticle(
+        raw,
+        "intro-dp",
+        "动态规划入门",
+        "# 动态规划入门\n\n动态规划的核心是：**状态定义**、**状态转移**、**边界条件**。\n",
+        now);
+  }
+
+  if (CountRows(raw, "kb_article_links") == 0) {
+    const auto p1 =
+        QueryOneId(raw, "SELECT id FROM problems WHERE slug='a-plus-b'");
+    const auto p2 =
+        QueryOneId(raw, "SELECT id FROM problems WHERE slug='fibonacci-n'");
+    const auto a1 =
+        QueryOneId(raw, "SELECT id FROM kb_articles WHERE slug='cpp-fast-io'");
+    const auto a2 =
+        QueryOneId(raw, "SELECT id FROM kb_articles WHERE slug='intro-dp'");
+    if (a1 && p1) InsertKbLink(raw, *a1, *p1);
+    if (a2 && p2) InsertKbLink(raw, *a2, *p2);
+  }
+
+  if (CountRows(raw, "contests") == 0) {
+    InsertContest(
+        raw,
+        "CSP 模拟赛（示例）",
+        now - 3600,
+        now + 7 * 24 * 3600,
+        R"({"type":"acm","desc":"按通过题数与罚时排名"})");
+  }
+
+  if (CountRows(raw, "contest_problems") == 0) {
+    const auto contest_id = QueryOneId(raw, "SELECT id FROM contests ORDER BY id LIMIT 1");
+    const auto p1 = QueryOneId(raw, "SELECT id FROM problems ORDER BY id LIMIT 1");
+    const auto p2 = QueryOneId(raw, "SELECT id FROM problems ORDER BY id LIMIT 1 OFFSET 1");
+    if (contest_id && p1) InsertContestProblem(raw, *contest_id, *p1, 1);
+    if (contest_id && p2) InsertContestProblem(raw, *contest_id, *p2, 2);
+  }
+
+  if (CountRows(raw, "redeem_items") == 0) {
+    InsertRedeemItem(
+        raw,
+        "私人玩游戏时间",
+        "全局用户可兑换：假期 1 小时 5 Rating；学习日/非节假日 1 小时 25 Rating。",
+        "小时",
+        5,
+        25,
+        1,
+        1,
+        0,
+        now);
+  }
+
+  // Always seed C++基础课程任务（幂等：按 slug 检测存在）。
+  {
+    const auto existing = QueryOneId(raw, "SELECT id FROM problems WHERE slug='cpp-basic-01-hello' LIMIT 1");
+    if (!existing.has_value()) {
+      const int64_t created = now;
+      struct CourseItem {
+        const char* slug;
+        const char* title;
+        int diff;
+        const char* source;
+        const char* md;
+        const char* tags[6];
+      };
+      const CourseItem items[] = {
+        {
+          "cpp-basic-01-hello",
+          "C++基础01：环境配置与Hello World（VSCode）",
+          1,
+          "course:cpp-basic:01",
+          R"MD(# C++基础01：环境配置与Hello World（VSCode）
+
+## 学习目标
+- 安装并打开 VSCode，创建并运行第一个 C++14 程序
+- 学会新建文件、保存、运行、查看输出
+- 了解 `main()`、`#include <iostream>`、`cout`
+
+## 推荐视频（观看后写笔记）
+- 保姆级：VSCode + mingw64 配置 C/C++（BV1tg411N7Fq）
+  - https://www.bilibili.com/video/BV1tg411N7Fq/
+- 每天五分钟学C++：01 开发工具（BV1dK4y137bk，系列入口）
+  - https://www.bilibili.com/video/BV1dK4y137bk/
+
+## 参考图文
+- LoongBa 极简配置：GCC/VSCode/HelloWorld
+  - https://github.com/LoongBa/Cpp_Beginner_Guide
+
+## 练习（完成至少 2 题）
+- B2002 Hello, World! https://www.luogu.com.cn/problem/B2002
+- P1000 超级玛丽游戏 https://www.luogu.com.cn/problem/P1000
+
+## 提交要求
+- 在本题页面下方“学习笔记”区域写下：
+  1) 你安装了什么、遇到什么坑、怎么解决
+  2) 你的 HelloWorld 代码
+  3) 你学到的 3 个关键词
+)MD",
+          {"cpp-basic", "vscode", "io", "", "", ""}
+        },
+        {
+          "cpp-basic-02-io",
+          "C++基础02：输入输出与变量",
+          1,
+          "course:cpp-basic:02",
+          R"MD(# C++基础02：输入输出与变量
+
+## 学习目标
+- 会用 `cin` 读入、`cout` 输出
+- 理解变量：`int / long long / double / char / string`
+
+## 推荐视频
+- 每天五分钟学C++：02 输出、03 变量（系列入口见上）
+  - https://www.bilibili.com/video/BV1dK4y137bk/
+
+## 练习
+- P1001 A+B Problem https://www.luogu.com.cn/problem/P1001
+- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008
+- P5704 字母转换 https://www.luogu.com.cn/problem/P5704
+
+## 提交要求
+- 上传/填写学习笔记：写出 `cin/cout` 模板、常见错误（空格/换行）
+)MD",
+          {"cpp-basic", "io", "types", "", "", ""}
+        },
+        {
+          "cpp-basic-03-branch",
+          "C++基础03：分支结构（if / switch）",
+          2,
+          "course:cpp-basic:03",
+          R"MD(# C++基础03：分支结构（if / switch）
+
+## 学习目标
+- 会写 `if / else if / else` 与基本逻辑运算
+- 能处理边界与分类讨论
+
+## 练习
+- B2035 判断数正负 https://www.luogu.com.cn/problem/B2035
+- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711
+- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909
+
+## 提交要求
+- 笔记里写清：你如何找“边界”（例如 0、最小/最大、等于条件）
+)MD",
+          {"cpp-basic", "branch", "logic", "", "", ""}
+        },
+        {
+          "cpp-basic-04-loop",
+          "C++基础04：循环结构（for / while）",
+          2,
+          "course:cpp-basic:04",
+          R"MD(# C++基础04：循环结构（for / while）
+
+## 学习目标
+- 会用循环做：计数、累加、打印图形
+
+## 练习
+- B2083 画矩形 https://www.luogu.com.cn/problem/B2083
+- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421
+
+## 提交要求
+- 笔记里写：循环三要素（初始化/条件/更新）+ 你调试的方法
+)MD",
+          {"cpp-basic", "loop", "debug", "", "", ""}
+        },
+        {
+          "cpp-basic-05-array",
+          "C++基础05：数组入门（一维）",
+          3,
+          "course:cpp-basic:05",
+          R"MD(# C++基础05：数组入门（一维）
+
+## 学习目标
+- 会定义数组、遍历、统计
+
+## 练习
+- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427
+- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428
+
+## 提交要求
+- 笔记里写：数组下标从 0/1 的选择；如何避免越界
+)MD",
+          {"cpp-basic", "array", "", "", "", ""}
+        }
+      };
+
+      for (const auto& it : items) {
+        InsertProblem(raw, it.slug, it.title, it.md, it.diff, it.source, "", "", created);
+        const auto pid = QueryOneId(raw, std::string("SELECT id FROM problems WHERE slug='") + it.slug + "' LIMIT 1");
+        if (pid.has_value()) {
+          for (const char* tag : it.tags) {
+            if (!tag || !*tag) continue;
+            InsertProblemTag(raw, *pid, tag);
+          }
+        }
+      }
+    }
+  }
+
+}
+
+}  // namespace csp::db

cpp_basic_20_upsert.sql

@@ -0,0 +1,363 @@
+BEGIN;
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-01-hello','C++基础01：环境配置与Hello World（VSCode）','# C++基础01：环境配置与Hello World（VSCode）
+
+## 学习目标
+- 安装并打开 VSCode，创建并运行第一个 C++14 程序
+- 学会新建文件、保存、运行、查看输出
+- 了解 `main()`、`#include <iostream>`、`cout`
+
+## 推荐视频（观看后写笔记）
+- 保姆级：VSCode + mingw64 配置 C/C++（BV1tg411N7Fq）
+  - https://www.bilibili.com/video/BV1tg411N7Fq/
+- 每天五分钟学C++：01 开发工具（BV1dK4y137bk，系列入口）
+  - https://www.bilibili.com/video/BV1dK4y137bk/
+
+## 参考图文
+- LoongBa 极简配置：GCC/VSCode/HelloWorld
+  - https://github.com/LoongBa/Cpp_Beginner_Guide
+
+## 练习（完成至少 2 题）
+- B2002 Hello, World! https://www.luogu.com.cn/problem/B2002
+- P1000 超级玛丽游戏 https://www.luogu.com.cn/problem/P1000
+
+## 提交要求
+- 在本题页面下方“学习笔记”区域写下：
+  1) 你安装了什么、遇到什么坑、怎么解决
+  2) 你的 HelloWorld 代码
+  3) 你学到的 3 个关键词
+',1,'course:cpp-basic:01',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-01-hello');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-01-hello'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-01-hello'),'vscode');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-01-hello'),'io');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-02-io','C++基础02：输入输出与变量','# C++基础02：输入输出与变量
+
+## 学习目标
+- 会用 `cin` 读入、`cout` 输出
+- 理解变量：`int / long long / double / char / string`
+
+## 推荐视频
+- 每天五分钟学C++：02 输出、03 变量（系列入口见上）
+  - https://www.bilibili.com/video/BV1dK4y137bk/
+
+## 练习
+- P1001 A+B Problem https://www.luogu.com.cn/problem/P1001
+- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008
+- P5704 字母转换 https://www.luogu.com.cn/problem/P5704
+
+## 提交要求
+- 上传/填写学习笔记：写出 `cin/cout` 模板、常见错误（空格/换行）
+',1,'course:cpp-basic:02',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-02-io');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-02-io'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-02-io'),'io');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-02-io'),'types');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-03-branch','C++基础03：分支结构（if / switch）','# C++基础03：分支结构（if / switch）
+
+## 学习目标
+- 会写 `if / else if / else` 与基本逻辑运算
+- 能处理边界与分类讨论
+
+## 练习
+- B2035 判断数正负 https://www.luogu.com.cn/problem/B2035
+- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711
+- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909
+
+## 提交要求
+- 笔记里写清：你如何找“边界”（例如 0、最小/最大、等于条件）
+',2,'course:cpp-basic:03',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-03-branch');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-03-branch'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-03-branch'),'branch');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-03-branch'),'logic');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-04-loop','C++基础04：循环结构（for / while）','# C++基础04：循环结构（for / while）
+
+## 学习目标
+- 会用循环做：计数、累加、打印图形
+
+## 练习
+- B2083 画矩形 https://www.luogu.com.cn/problem/B2083
+- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421
+
+## 提交要求
+- 笔记里写：循环三要素（初始化/条件/更新）+ 你调试的方法
+',2,'course:cpp-basic:04',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-04-loop');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-04-loop'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-04-loop'),'loop');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-04-loop'),'debug');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-05-array','C++基础05：数组入门（一维）','# C++基础05：数组入门（一维）
+
+## 学习目标
+- 会定义数组、遍历、统计
+
+## 练习
+- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427
+- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428
+
+## 提交要求
+- 笔记里写：数组下标从 0/1 的选择；如何避免越界
+',3,'course:cpp-basic:05',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-05-array');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-05-array'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-05-array'),'array');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-06-char-string','C++基础06：字符与字符串（char / string）','# C++基础06：字符与字符串（char / string）
+
+## 学习目标
+- 区分 `char`（一个字符）与 `string`（一串字符）
+- 会做：大小写转换、统计字母、简单加密
+
+## 重点
+- `char c=''A''`，`string s="abc"`
+- `s.size()` / `s[i]` / 遍历字符串
+
+## 练习（完成至少 2 题）
+- P5704 字母转换 https://www.luogu.com.cn/problem/P5704
+- P5733 自动修正 https://www.luogu.com.cn/problem/P5733
+- P1914 小书童——凯撒密码 https://www.luogu.com.cn/problem/P1914
+
+## 提交要求
+- 笔记写：`string` 下标访问与越界风险（最后一个下标是 size()-1）
+',3,'course:cpp-basic:06',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-06-char-string');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-06-char-string'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-06-char-string'),'string');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-06-char-string'),'char');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-07-operator-cast','C++基础07：运算、取整与类型转换（/ %）','# C++基础07：运算、取整与类型转换（/ %）
+
+## 学习目标
+- 写对 `+ - * / %`
+- 理解：整数除法会向下取整；`%` 只能用于整数
+- 学会用 `long long` 防止溢出
+
+## 练习
+- B2008 计算 (a+b)×c 的值 https://www.luogu.com.cn/problem/B2008
+- P1421 小玉买文具 https://www.luogu.com.cn/problem/P1421
+- P5708 三角形面积 https://www.luogu.com.cn/problem/P5708
+
+## 提交要求
+- 笔记写：什么时候要用 `long long`（例如钱、人数、乘法很大）
+',3,'course:cpp-basic:07',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-07-operator-cast');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-07-operator-cast'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-07-operator-cast'),'math');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-07-operator-cast'),'cast');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-08-logic','C++基础08：逻辑运算与边界（&& || !）','# C++基础08：逻辑运算与边界（&& || !）
+
+## 学习目标
+- 能把题目里的“并且/或者/不是”写成条件
+- 学会找边界：等于、最小/最大、0
+
+## 练习
+- P5710 数的性质 https://www.luogu.com.cn/problem/P5710
+- P5711 闰年判断 https://www.luogu.com.cn/problem/P5711
+- P1909 买铅笔 https://www.luogu.com.cn/problem/P1909
+
+## 提交要求
+- 笔记写：你列出的边界测试用例（至少 3 个）
+',3,'course:cpp-basic:08',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-08-logic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-08-logic'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-08-logic'),'logic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-08-logic'),'boundary');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-09-switch','C++基础09：switch 与枚举列情况','# C++基础09：switch 与枚举列情况
+
+## 学习目标
+- 会写 `switch/case/break`
+- 面对“情况很少”的题，会列举所有情况
+
+## 练习
+- P2433 小学数学N合一 https://www.luogu.com.cn/problem/P2433
+
+## 提交要求
+- 笔记写：为什么每个 case 后要 `break`（避免贯穿执行）
+',3,'course:cpp-basic:09',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-09-switch');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-09-switch'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-09-switch'),'switch');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-09-switch'),'enum');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-10-for','C++基础10：for循环（计数与累加）','# C++基础10：for循环（计数与累加）
+
+## 学习目标
+- 熟练掌握 `for(初始化; 条件; 更新)`
+- 会写：求和、求最大最小、统计个数
+
+## 练习
+- P1425 小鱼的游泳时间 https://www.luogu.com.cn/problem/P1425
+- P5718 找最小值 https://www.luogu.com.cn/problem/P5718
+
+## 提交要求
+- 笔记写：循环“三要素”分别是什么
+',3,'course:cpp-basic:10',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-10-for');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-10-for'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-10-for'),'loop');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-10-for'),'for');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-11-while','C++基础11：while循环（未知次数）','# C++基础11：while循环（未知次数）
+
+## 学习目标
+- 会用 `while` 处理：读到结束、读到 0 停止
+- 避免死循环：每次循环必须改变状态
+
+## 练习
+- P1427 小鱼的数字游戏 https://www.luogu.com.cn/problem/P1427
+- P1047 校门外的树（选做）https://www.luogu.com.cn/problem/P1047
+
+## 提交要求
+- 笔记写：你是如何避免死循环的（更新变量写在哪里）
+',3,'course:cpp-basic:11',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-11-while');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-11-while'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-11-while'),'loop');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-11-while'),'while');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-12-nested','C++基础12：嵌套循环与打印图形','# C++基础12：嵌套循环与打印图形
+
+## 学习目标
+- 外层循环=行，内层循环=列
+- 能打印：矩形、三角形、乘法表
+
+## 练习
+- B2083 画矩形 https://www.luogu.com.cn/problem/B2083
+- P5717 三角形分类（练分类讨论 + 输出格式）https://www.luogu.com.cn/problem/P5717
+
+## 提交要求
+- 笔记写：你画图时是怎么想“行/列”的
+',3,'course:cpp-basic:12',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-12-nested');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-12-nested'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-12-nested'),'nested-loop');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-12-nested'),'print');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-13-array-ops','C++基础13：数组进阶（统计/查找/反转）','# C++基础13：数组进阶（统计/查找/反转）
+
+## 学习目标
+- 用数组做：统计、查找位置、反转、去重（了解）
+
+## 练习
+- P1428 小鱼比可爱 https://www.luogu.com.cn/problem/P1428
+- P5727 冰雹猜想 https://www.luogu.com.cn/problem/P5727
+
+## 提交要求
+- 笔记写：数组越界的 2 个典型原因 + 你的检查方法
+',4,'course:cpp-basic:13',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-13-array-ops');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-13-array-ops'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-13-array-ops'),'array');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-13-array-ops'),'practice');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-14-function','C++基础14：函数入门（把重复代码变成工具）','# C++基础14：函数入门（参数/返回值/作用域）
+
+## 学习目标
+- 会定义并调用函数
+- 知道局部变量作用域（{} 内有效）
+
+## 练习
+- P5735 距离函数 https://www.luogu.com.cn/problem/P5735
+- P5739 计算阶乘（也可用递归）https://www.luogu.com.cn/problem/P5739
+
+## 提交要求
+- 笔记写：你把哪段重复代码抽成了函数？为什么这样更清晰？
+',4,'course:cpp-basic:14',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-14-function');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-14-function'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-14-function'),'function');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-14-function'),'scope');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-15-recursion','C++基础15：递归入门（选学）','# C++基础15：递归入门（选学）
+
+## 学习目标
+- 理解递归：函数调用自己
+- 必须有终止条件（否则会无限调用）
+
+## 练习
+- P5739 计算阶乘 https://www.luogu.com.cn/problem/P5739
+- P1028 数的计算（选做）https://www.luogu.com.cn/problem/P1028
+
+## 提交要求
+- 笔记写：你的递归“终止条件”是什么？
+',4,'course:cpp-basic:15',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-15-recursion');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-15-recursion'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-15-recursion'),'recursion');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-16-struct','C++基础16：结构体 struct（选学）','# C++基础16：结构体 struct（选学）
+
+## 学习目标
+- 用 `struct` 把多个字段打包成一个“人物卡/记录”
+- 学会 `struct` 数组
+
+## 练习
+- P5740 最厉害的学生 https://www.luogu.com.cn/problem/P5740
+- P5744 培训 https://www.luogu.com.cn/problem/P5744
+
+## 提交要求
+- 笔记写：struct 适合用在什么场景（多个属性属于同一个对象）
+',4,'course:cpp-basic:16',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-16-struct');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-16-struct'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-16-struct'),'struct');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-17-sort','C++基础17：排序初步（sort）','# C++基础17：排序初步（sort）
+
+## 学习目标
+- 会用 `#include <algorithm>` 和 `sort`
+- 排序后做统计/查找会更简单
+
+## 练习
+- P1059 明明的随机数 https://www.luogu.com.cn/problem/P1059
+- P1093 奖学金（选做）https://www.luogu.com.cn/problem/P1093
+
+## 提交要求
+- 笔记写：sort 的区间是 `[begin, end)`（end 不包含）
+',4,'course:cpp-basic:17',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-17-sort');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-17-sort'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-17-sort'),'sort');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-17-sort'),'algorithm');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-18-binary-search','C++基础18：二分查找初步（lower_bound）','# C++基础18：二分查找初步（lower_bound）
+
+## 学习目标
+- 理解“在有序数组里快速找答案”
+- 会用 `lower_bound` 找第一个 >= x 的位置
+
+## 练习
+- P2249 查找 https://www.luogu.com.cn/problem/P2249
+
+## 提交要求
+- 笔记写：二分查找的前提：数组必须有序
+',4,'course:cpp-basic:18',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-18-binary-search');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-18-binary-search'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-18-binary-search'),'binary-search');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-19-prefix-sum','C++基础19：前缀和（区间求和神器）','# C++基础19：前缀和（区间求和神器）
+
+## 学习目标
+- 会构造前缀和 `s[i]=s[i-1]+a[i]`
+- 会算区间和 `sum(l,r)=s[r]-s[l-1]`
+
+## 练习
+- P8218 求区间和（前缀和基础）https://www.luogu.com.cn/problem/P8218
+
+## 提交要求
+- 笔记写：下标偏移是怎么处理的（从 1 开始更方便）
+',4,'course:cpp-basic:19',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-19-prefix-sum');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-19-prefix-sum'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-19-prefix-sum'),'prefix-sum');
+INSERT INTO problems(slug,title,statement_md,difficulty,source,created_at) VALUES('cpp-basic-20-simulation','C++基础20：模拟题与综合复盘','# C++基础20：模拟题与综合复盘
+
+## 学习目标
+- 学会按题意一步步做（模拟）
+- 复盘：总结常见错误 + 常用模板
+
+## 练习（从这里选 3~5 题做小测）
+- P1957 口算练习题 https://www.luogu.com.cn/problem/P1957
+- P1055 ISBN号码 https://www.luogu.com.cn/problem/P1055
+- P2433 小学数学N合一（回顾）https://www.luogu.com.cn/problem/P2433
+
+## 提交要求
+- 笔记写：
+  1) 你最常犯的 3 类错误
+  2) 你最常用的 5 行代码模板
+  3) 下周你准备怎么练
+',4,'course:cpp-basic:20',1771241843) ON CONFLICT(slug) DO UPDATE SET title=excluded.title, statement_md=excluded.statement_md, difficulty=excluded.difficulty, source=excluded.source;
+DELETE FROM problem_tags WHERE problem_id=(SELECT id FROM problems WHERE slug='cpp-basic-20-simulation');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-20-simulation'),'cpp-basic');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-20-simulation'),'simulation');
+INSERT OR IGNORE INTO problem_tags(problem_id,tag) VALUES((SELECT id FROM problems WHERE slug='cpp-basic-20-simulation'),'review');
+COMMIT;

frontend/src/app/problems/[id]/page.tsx

@@ -950,14 +950,18 @@ export default function ProblemDetailPage() {
 
   const printProblemWithAnswer = async () => {
     let resolved = solutionData;
-    if (resolved?.access?.mode !== "full") {
+    // If user has already unlocked solutions (full mode), use them directly
-      const fetched = await loadSolutions("full");
+    if (resolved?.access?.mode === "full" && (resolved?.items.length ?? 0) > 0) {
-      if (fetched) resolved = fetched;
+      // Already have full solutions loaded — use as-is
-    } else if ((resolved?.items.length ?? 0) === 0) {
+    } else if (resolved?.access?.mode === "full") {
+      // Full access but items not loaded yet — fetch
       const fetched = await loadSolutions("full");
       if (fetched) resolved = fetched;
     }
-    setPrintAnswerMarkdown(buildPrintableAnswerMarkdown(resolved, tx));
+    // If not in full mode, just print problem without answer
+    setPrintAnswerMarkdown(
+      resolved?.access?.mode === "full" ? buildPrintableAnswerMarkdown(resolved, tx) : ""
+    );
     await new Promise<void>((resolve) => {
       window.requestAnimationFrame(() => {
         window.requestAnimationFrame(() => resolve());
@@ -997,7 +1001,11 @@ export default function ProblemDetailPage() {
                 onClick={() => void printProblemWithAnswer()}
                 disabled={solutionLoading}
               >
-                {solutionLoading ? tx("卷轴准备中...", "Scribing...") : tx("打印卷轴", "Scribe Scroll")}
+                {solutionLoading
+                  ? tx("卷轴准备中...", "Scribing...")
+                  : solutionData?.access?.mode === "full"
+                    ? tx("🖨️ 打印题目+答案", "🖨️ Print with Answer")
+                    : tx("🖨️ 打印题目", "🖨️ Print Problem")}
               </button>
               <button
                 className="mc-btn text-sm py-1"

remote_patch_cpp_basic_20.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+from __future__ import annotations
+
+import argparse
+from pathlib import Path
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument('--file', required=True)
+    ap.add_argument('--items', required=True)
+    args = ap.parse_args()
+
+    p = Path(args.file)
+    items = Path(args.items).read_text(encoding='utf-8')
+    s = p.read_text(encoding='utf-8')
+
+    marker = 'const CourseItem items[] = {'
+    i = s.find(marker)
+    if i < 0:
+        raise SystemExit(f'cannot find marker: {marker}')
+
+    j = s.find('};', i)
+    if j < 0:
+        raise SystemExit('cannot find end marker "};" after items start')
+
+    before = s[:i]
+    after = s[j+2:]
+
+    out = before + items + after
+    if out == s:
+        print('no change')
+        return
+
+    backup = p.with_suffix(p.suffix + '.bak_cppbasic20')
+    backup.write_text(s, encoding='utf-8')
+    p.write_text(out, encoding='utf-8')
+    print('patched', str(p))
+    print('backup', str(backup))
+
+
+if __name__ == '__main__':
+    main()