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/ai_assistant.py

@@ -9,6 +9,7 @@ from app.ai.local_faq import match_faq
 from app.api.v1.deps import get_current_user_id
 from app.models.system_config import SystemConfig
 from app.services.admin import AdminService
+from app.services.credit import CreditService
 import logging
 import time
 import re
@@ -108,6 +109,14 @@ async def chat(
             f"db={t1-t0:.2f}s orm={t2-t1:.2f}s total={t4-t_start:.2f}s"
         )
     else:
+        credit_svc = CreditService(db)
+        ok, balance = await credit_svc.deduct(user_id, "ai_chat")
+        if not ok:
+            raise HTTPException(
+                status_code=402,
+                detail=f"次数不足 (剩余 {balance:.1f})"
+            )
+
         t3 = time.time()
         ai = get_ai_router()
         result = await ai.chat(data.message, data.history or [], system_prompt)