feat(domain): add entities and json helpers for sqlite schema

backend/include/csp/domain/entities.h

@@ -0,0 +1,84 @@
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <string>
+
+namespace csp::domain {
+
+// Notes:
+// - All *_at fields use Unix timestamp seconds (int64).
+// - id fields match SQLite INTEGER PRIMARY KEY AUTOINCREMENT (int64).
+
+struct User {
+  int64_t id = 0;
+  std::string username;
+  std::string password_salt;
+  std::string password_hash;
+  int32_t rating = 0;
+  int64_t created_at = 0;
+};
+
+struct Session {
+  std::string token;  // PK
+  int64_t user_id = 0;
+  int64_t expires_at = 0;
+  int64_t created_at = 0;
+};
+
+struct Problem {
+  int64_t id = 0;
+  std::string slug;
+  std::string title;
+  std::string statement_md;
+  int32_t difficulty = 1;
+  std::string source;
+  int64_t created_at = 0;
+};
+
+struct ProblemTag {
+  int64_t problem_id = 0;
+  std::string tag;
+};
+
+enum class SubmissionStatus {
+  Pending,
+  Compiling,
+  Running,
+  AC,
+  WA,
+  TLE,
+  MLE,
+  RE,
+  CE,
+  Unknown,
+};
+
+// MVP: only C++ is supported.
+enum class Language {
+  Cpp,
+  Unknown,
+};
+
+struct Submission {
+  int64_t id = 0;
+  int64_t user_id = 0;
+  int64_t problem_id = 0;
+  Language language = Language::Cpp;
+  std::string code;
+  SubmissionStatus status = SubmissionStatus::Pending;
+  int32_t score = 0;
+  int32_t time_ms = 0;
+  int32_t memory_kb = 0;
+  int64_t created_at = 0;
+};
+
+struct WrongBookItem {
+  int64_t user_id = 0;
+  int64_t problem_id = 0;
+  std::optional<int64_t> last_submission_id;
+  std::string note;
+  int64_t updated_at = 0;
+};
+
+}  // namespace csp::domain

backend/include/csp/domain/enum_strings.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "csp/domain/entities.h"
+
+#include <string>
+
+namespace csp::domain {
+
+// String mapping for persistence/API.
+
+std::string ToString(SubmissionStatus s);
+SubmissionStatus SubmissionStatusFromString(const std::string& s);
+
+std::string ToString(Language l);
+Language LanguageFromString(const std::string& s);
+
+}  // namespace csp::domain

backend/include/csp/domain/json.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include "csp/domain/entities.h"
+#include "csp/domain/enum_strings.h"
+
+#include <json/json.h>
+
+namespace csp::domain {
+
+// These helpers are intended for API responses.
+// IMPORTANT: They intentionally exclude sensitive fields (password_salt/password_hash).
+
+Json::Value ToPublicJson(const User& u);
+Json::Value ToJson(const Problem& p);
+Json::Value ToJson(const Submission& s);
+Json::Value ToJson(const WrongBookItem& w);
+
+}  // namespace csp::domain