fix(blackjack): return db_error on bet-commit failure; document new patterns

- do_blackjack_bet now wraps its _commit so a DB failure returns db_error
  (handled by the command layer) instead of raising through the interaction.
- DEV_NOTES "Adding a New Economy Command" checklist updated: use
  effective_cooldown / store.ITEM_COOLDOWNS as the single source of truth for
  item-modified cooldowns, and handle db_error via reply_db_error.

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

core/economy/gambling.py

@@ -248,7 +248,12 @@ async def do_blackjack_bet(user_id: int, bet: int) -> dict:
     if user["balance"] < bet:
         return {"ok": False, "reason": "insufficient", "balance": user["balance"]}
     user["balance"] -= bet
-    await _commit(user_id, user)
+    try:
+        await _commit(user_id, user)
+    except DatabaseError:
+        # Deduction never persisted, so the player was not charged - report it
+        # instead of raising through the interaction handler.
+        return {"ok": False, "reason": "db_error"}
     return {"ok": True, "balance": user["balance"]}