feat: add streak reward and 100-achievement milestone system

backend/src/db/sqlite_db.cc

@@ -930,6 +930,16 @@ CREATE TABLE IF NOT EXISTS source_crystal_transactions (
   FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE
 );
 
+CREATE TABLE IF NOT EXISTS user_achievement_milestones (
+  user_id INTEGER NOT NULL,
+  milestone_no INTEGER NOT NULL,
+  completed_count_snapshot INTEGER NOT NULL DEFAULT 0,
+  rating_bonus INTEGER NOT NULL DEFAULT 20,
+  created_at INTEGER NOT NULL,
+  PRIMARY KEY(user_id, milestone_no),
+  FOREIGN KEY(user_id) REFERENCES users(id) ON DELETE CASCADE
+);
+
 CREATE TABLE IF NOT EXISTS daily_task_logs (
   id INTEGER PRIMARY KEY AUTOINCREMENT,
   user_id INTEGER NOT NULL,
@@ -1049,6 +1059,7 @@ 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_user_experience_logs_user_created ON user_experience_logs(user_id, created_at DESC);
 CREATE INDEX IF NOT EXISTS idx_source_crystal_tx_user_created ON source_crystal_transactions(user_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_user_achievement_milestones_user_created ON user_achievement_milestones(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");