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/.env.example

@@ -57,13 +57,11 @@ WECHAT_APP_ID=
 WECHAT_APP_SECRET=
 WECHAT_PUSH_TEMPLATE_ID=
 
-# 微信支付
-WECHAT_PAY_MCH_ID=
-WECHAT_PAY_API_KEY=
-WECHAT_PAY_SERIAL_NO=
-WECHAT_PAY_CERT_DIR=./certs
-WECHAT_PAY_NOTIFY_URL=https://your-domain.com/api/v1/payment/notify
-WECHAT_PAY_API_BASE=https://api.mch.weixin.qq.com
+# 统一支付网关（宇之然 pay-api，支持支付宝/微信）
+PAY_API_KEY=pay_98c86e0d2eba4379bfe722c8
+PAY_API_SECRET=cc392f42daf94719b9b157f3e7ad6c9472ae20a33ba14323
+PAY_API_BASE_URL=https://www.yzrcloud.cn/api/gateway
+PAY_WEBHOOK_URL=https://your-domain.com/api/v1/payment/webhook
 
 # 汇率 API（免费层即可）
 EXCHANGE_RATE_API_KEY=