refactor: replace direct WeChat/Alipay with unified pay-api gateway

Switch from direct WeChat Pay / Alipay integrations to the unified
宇之然 pay-api gateway (HMAC-SHA256 auth). Removes wechat_pay.py,
keeps PaymentGateway abstraction, adds UnifiedPayService. Simplifies
payment.py create_order to {plan, pay_type} params. Single webhook
endpoint replaces separate WeChat/Alipay notify handlers.

backend/app/models/payment_transaction.py

@@ -0,0 +1,29 @@
+from sqlalchemy import Column, String, Integer, DateTime, Float, Text, Boolean
+from sqlalchemy.dialects.postgresql import UUID
+from datetime import datetime
+from app.database import Base
+import uuid
+
+
+class PaymentTransaction(Base):
+    __tablename__ = "payment_transactions"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    user_id = Column(UUID(as_uuid=True), nullable=False, index=True)
+    order_no = Column(String(64), unique=True, nullable=False, index=True)
+    gateway_order_id = Column(String(128), nullable=True)
+    gateway_order_no = Column(String(128), nullable=True)
+    plan = Column(String(50), nullable=False)
+    amount = Column(Float, nullable=False)
+    currency = Column(String(10), default="CNY")
+    gateway = Column(String(20), nullable=False)
+    pay_type = Column(String(20), nullable=False)
+    status = Column(String(20), default="pending")
+    description = Column(Text, nullable=True)
+    refund_amount = Column(Float, default=0)
+    refund_reason = Column(Text, nullable=True)
+    paid_at = Column(DateTime, nullable=True)
+    refunded_at = Column(DateTime, nullable=True)
+    notify_raw = Column(Text, nullable=True)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)