feat(backend): add sqlite migrations + app state + tests

backend/migrations/001_init.sql

@@ -0,0 +1,66 @@
+-- 001_init.sql
+PRAGMA foreign_keys = ON;
+
+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 "",
+  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,
+  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,
+  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 wrong_book (
+  user_id INTEGER NOT NULL,
+  problem_id INTEGER NOT NULL,
+  last_submission_id INTEGER,
+  note TEXT NOT NULL DEFAULT "",
+  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 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);