feat(economy): add /achievements milestone badges

15 one-time achievements over the lifetime stat counters (work/wealth/gambling/
crime/heists/fishing/streaks/prestige), each paying a modest one-time coin reward
when unlocked. Detection is lazy - opening /achievements claims any newly earned
(like quests roll on view) - so no per-command hook is needed, and rewards are a
bounded, one-time coin source.

- New achievements.py: ACHIEVEMENTS table, pure newly_earned/achievements_view,
  and locked do_check_achievements. New achievements_earned schema field.
- /achievements command (own view claims; viewing others is read-only) with
  progress bars, db_error handling, and an "unlocked" banner.

8 tests: threshold detection, one-time claim (no double pay), multi-unlock,
view progress capping.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013VbAVsrZuYesea99mPMmPT

core/economy/store.py

@@ -154,6 +154,7 @@ class UserData(TypedDict, total=False):
     total_received: int
     best_daily_streak: int
     lootboxes_opened: int
+    achievements_earned: list         # ids of achievements already claimed
     heist_global_cd_until: float
     # Prestige system
     prestige_level: int
@@ -217,6 +218,7 @@ def _default_user() -> UserData:
         "total_received":  0,
         "best_daily_streak": 0,
         "lootboxes_opened": 0,
+        "achievements_earned": [],
         "heist_global_cd_until": 0.0,
         # ── Prestige ─────────────────────────────────────────────────────────
         "prestige_level":    0,