Phase 1: database schema, migrations, repository, user seeding

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/schema.sql

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+CREATE TABLE IF NOT EXISTS users (
+  id INTEGER PRIMARY KEY,
+  name TEXT UNIQUE NOT NULL,
+  password_hash TEXT NOT NULL,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS questions (
+  id TEXT PRIMARY KEY,
+  text TEXT NOT NULL,
+  source TEXT,
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS answers (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  question_id TEXT NOT NULL REFERENCES questions(id) ON DELETE CASCADE,
+  text TEXT NOT NULL,
+  is_correct BOOLEAN NOT NULL,
+  position INTEGER NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS tests (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id INTEGER NOT NULL REFERENCES users(id),
+  created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+  completed_at DATETIME,
+  n_questions INTEGER NOT NULL,
+  question_ids TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS test_answers (
+  test_id INTEGER NOT NULL REFERENCES tests(id) ON DELETE CASCADE,
+  question_id TEXT NOT NULL REFERENCES questions(id),
+  selected_answer_id INTEGER REFERENCES answers(id),
+  is_correct BOOLEAN,
+  answered_at DATETIME,
+  PRIMARY KEY (test_id, question_id)
+);
+
+CREATE TABLE IF NOT EXISTS user_question_stats (
+  user_id INTEGER NOT NULL REFERENCES users(id),
+  question_id TEXT NOT NULL REFERENCES questions(id) ON DELETE CASCADE,
+  times_seen INTEGER NOT NULL DEFAULT 0,
+  times_correct INTEGER NOT NULL DEFAULT 0,
+  last_seen_at DATETIME,
+  PRIMARY KEY (user_id, question_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_test_answers_test ON test_answers(test_id);
+CREATE INDEX IF NOT EXISTS idx_answers_question ON answers(question_id);
+CREATE INDEX IF NOT EXISTS idx_stats_user ON user_question_stats(user_id);