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/user_credit.py

@@ -0,0 +1,26 @@
+from sqlalchemy import Column, String, Float, Boolean, DateTime, ForeignKey, Integer, Date
+from sqlalchemy.dialects.postgresql import UUID
+from datetime import datetime
+from app.database import Base
+import uuid
+
+
+class UserCredit(Base):
+    __tablename__ = "user_credits"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    user_id = Column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=False, unique=True, index=True)
+    balance = Column(Float, default=0)
+    total_purchased = Column(Float, default=0)
+    total_used = Column(Float, default=0)
+
+    subscription_plan_id = Column(UUID(as_uuid=True), ForeignKey("subscription_plans.id"), nullable=True)
+    subscription_expires_at = Column(DateTime, nullable=True)
+    subscription_auto_renew = Column(Boolean, default=False)
+
+    free_trial_used = Column(Boolean, default=False)
+    daily_translate_chars = Column(Integer, default=0)
+    daily_translate_date = Column(Date, nullable=True)
+
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)