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/translate.py

@@ -6,6 +6,7 @@ from app.database import get_db
 from app.services.translation import TranslationService
 from app.services.tts import tts_service
 from app.services.preference import UserPreferenceService
+from app.services.credit import CreditService
 from app.core.security import decode_token
 from app.api.v1.deps import get_current_user_id
 
@@ -35,6 +36,7 @@ class ExtractRequest(BaseModel):
 async def translate_text(
     data: TranslateRequest,
     user_id: str = Depends(get_current_user_id),
+    db: AsyncSession = Depends(get_db),
 ):
     service = TranslationService()
     result = await service.translate(
@@ -44,6 +46,13 @@ async def translate_text(
         context=data.context,
         user_id=user_id,
     )
+
+    credit_svc = CreditService(db)
+    char_count = len(data.text)
+    await credit_svc.deduct(
+        user_id, "translate",
+        metadata={"chars": char_count, "target_lang": data.target_lang},
+    )
     return result
 
 
@@ -54,6 +63,15 @@ async def generate_reply(
     db: AsyncSession = Depends(get_db),
 ):
     pref_service = UserPreferenceService(db)
+
+    credit_svc = CreditService(db)
+    ok, balance = await credit_svc.deduct(user_id, "reply_suggest")
+    if not ok:
+        raise HTTPException(
+            status_code=402,
+            detail=f"次数不足 (剩余 {balance:.1f}, 需要 2)"
+        )
+
     pref_context = await pref_service.get_preference_context(user_id, "reply")
 
     service = TranslationService()
@@ -71,7 +89,16 @@ async def generate_reply(
 async def extract_info(
     data: ExtractRequest,
     user_id: str = Depends(get_current_user_id),
+    db: AsyncSession = Depends(get_db),
 ):
+    credit_svc = CreditService(db)
+    ok, balance = await credit_svc.deduct(user_id, "info_extract")
+    if not ok:
+        raise HTTPException(
+            status_code=402,
+            detail=f"次数不足 (剩余 {balance:.1f}, 需要 1)"
+        )
+
     service = TranslationService()
     result = await service.extract_info(data.text, data.extract_type)
     return {"extracted": result, "type": data.extract_type}