feat: credit-based billing system

- New DB models: credit_packages, subscription_plans, user_credits, credit_consumptions, credit_purchases
- CreditService: balance, deduct, add_credits, grant_free_trial, history
- User API: /api/v1/credits/* (balance/history/packages/purchase/subscribe)
- Admin API: /api/v1/admin/credit-* (CRUD packages/plans, user credits, consumptions)
- PaymentService.create_credit_order + handle_callback for credit purchases
- Credit deduction on: discovery, translate, marketing, ai_chat, followup
- Free trial 30 credits on registration
- Documentation: docs/CREDIT_SYSTEM.md

backend/app/models/__init__.py

@@ -19,6 +19,10 @@ from .search_provider import SearchProvider
 from .discovery_record import DiscoveryRecord
 from .ai_provider import AIProvider
 from .payment_transaction import PaymentTransaction
+from .credit_package import CreditPackage, SubscriptionPlan
+from .user_credit import UserCredit
+from .credit_consumption import CreditConsumption
+from .credit_purchase import CreditPurchase
 
 __all__ = [
     "User", "Product",
@@ -37,4 +41,8 @@ __all__ = [
     "DiscoveryRecord",
     "AIProvider",
     "PaymentTransaction",
+    "CreditPackage", "SubscriptionPlan",
+    "UserCredit",
+    "CreditConsumption",
+    "CreditPurchase",
 ]