question packs instead of monthly/yearly plans; hearts are real

- packs (20 / 60 / 150 questions for 45,000 / 119,000 / 259,000 toman) never expire and
  give 7 / 30 / 60 days of unlimited hearts; old subscription plans stay for existing
  subscribers but are no longer on sale
- credit ledger (subscription.QuestionCredit): +N per paid pack (once per order), -1 per
  tutor question that reaches the API; the free daily questions are spent first; questions
  stopped by the server filter, cached answers and failed answers cost nothing
- unlimited hearts from packs live in HeartsPass, so buying a pack never rewrites a
  running subscription
- Message.charge records how each question was paid; GET /api/ai/quota and the ask
  responses report free-left-today and credits
- HEARTS_ENFORCED now defaults to on: zero hearts stops answering until refill or a pack

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

payment/services.py

@@ -176,6 +176,10 @@ def apply_order_payment_result(*, order: Order, gateway: str, gateway_ref_id: st
             if order.subscription_id:
                 from subscription.services import activate_subscription
                 activate_subscription(order.subscription_id, plan_id=order.plan_id)
+            elif order.plan_id:
+                # بسته‌ی سوال: اعتبار و روزهای قلبِ نامحدود، بی‌دست زدن به اشتراک.
+                from subscription.services import grant_pack
+                grant_pack(order)
             # پورسانتِ معرف فقط روی پرداختِ واقعی (نه ثبت‌نام، نه مهمان).
             from affiliate.services import record_paid_order
             record_paid_order(order)