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/api/v1/auth.py

@@ -81,6 +81,10 @@ async def register(
     )
     db.add(sub)
 
+    from app.services.credit import CreditService
+    credit_svc = CreditService(db)
+    await credit_svc.grant_free_trial(user.id)
+
     if data.ref_code:
         try:
             from app.api.v1.referral import do_claim_referral