Phase 24: daily check-in system + club milestones endpoint

- migrations/0012_daily_checkin.sql: daily_checkins table tracking streak,
  coins awarded, pack granted, timestamp per profile
- services/checkin.rs: get_status() (available, streak_day, next reward),
  claim() (idempotent same-day guard, streak logic: continue if yesterday
  or today, else reset; 7-day cycle with STREAK_COINS array, day-7 pack)
- routes/club.rs: GET /club/checkin, POST /club/checkin, GET /club/milestones
  (computed from statistics, season_history, owned_cards, sbc_submissions,
  daily_checkins tables; no new DB tables needed)
- 4 new integration tests: checkin available initially, claim awards coins,
  idempotent same-day, milestones endpoint structure (93 → 93+4=97 tests)

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

src/services/mod.rs

@@ -1,5 +1,6 @@
 pub mod achievement;
 pub mod card_db;
+pub mod checkin;
 pub mod club;
 pub mod notification;
 pub mod draft;