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/credit_package.py

@@ -0,0 +1,37 @@
+from sqlalchemy import Column, String, Integer, Float, Boolean, DateTime
+from sqlalchemy.dialects.postgresql import UUID
+from datetime import datetime
+from app.database import Base
+import uuid
+
+
+class CreditPackage(Base):
+    __tablename__ = "credit_packages"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name = Column(String(100), nullable=False)
+    name_en = Column(String(100), nullable=False)
+    credits = Column(Integer, nullable=False)
+    price = Column(Float, nullable=False)
+    price_usd = Column(Float)
+    original_price = Column(Float)
+    is_active = Column(Boolean, default=True)
+    sort_order = Column(Integer, default=0)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
+
+class SubscriptionPlan(Base):
+    __tablename__ = "subscription_plans"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name = Column(String(100), nullable=False)
+    name_en = Column(String(100), nullable=False)
+    credits_per_month = Column(Integer, nullable=False)
+    price = Column(Float, nullable=False)
+    price_usd = Column(Float)
+    duration_days = Column(Integer, default=30)
+    is_active = Column(Boolean, default=True)
+    sort_order = Column(Integer, default=0)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)