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/services/payment_gateway.py

@@ -0,0 +1,36 @@
+from abc import ABC, abstractmethod
+from typing import Optional, Dict, Any
+
+
+class PaymentGateway(ABC):
+    name: str = ""
+
+    @abstractmethod
+    async def create_order(self, order_no: str, amount: int, description: str,
+                           **kwargs) -> Dict[str, Any]:
+        ...
+
+    @abstractmethod
+    async def query_order(self, order_no: str) -> Dict[str, Any]:
+        ...
+
+    @abstractmethod
+    async def refund(self, order_no: str, amount: int, reason: str = "") -> Dict[str, Any]:
+        ...
+
+    @abstractmethod
+    async def query_refund(self, order_no: str) -> Dict[str, Any]:
+        ...
+
+    @abstractmethod
+    def verify_callback(self, headers: dict, body: str) -> bool:
+        ...
+
+    @abstractmethod
+    def parse_callback(self, body: str, headers: dict) -> Dict[str, Any]:
+        ...
+
+    def supports(self, pay_type: str) -> bool:
+        return pay_type in self.supported_types
+
+    supported_types: list = []