feat: add streak reward and 100-achievement milestone system

backend/migrations/001_init.sql

@@ -379,6 +379,16 @@ 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
+);
+
 INSERT OR IGNORE INTO source_crystal_settings(id, monthly_interest_rate, updated_at)
 VALUES(1, 0.02, strftime('%s','now'));
 
@@ -447,3 +457,4 @@ CREATE INDEX IF NOT EXISTS idx_problem_solution_jobs_problem ON problem_solution
 CREATE INDEX IF NOT EXISTS idx_problem_solutions_problem ON problem_solutions(problem_id, variant, id);
 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);