移除摸大红支付后建群逻辑，仅保留扫码与复制

支付成功只固化客服二维码与订单文案；同步清理迁移、配置与前后端相关字段。

backend/internal/router/router.go

@@ -259,7 +259,7 @@ func New(cfg config.Config, deps Dependencies, logger *zap.Logger) *gin.Engine {
 	var mohongService *mohong.Service
 	var mohongHandler *mohong.Handler
 	if deps.DB != nil {
-		mohongRepo = mohong.NewRepository(deps.DB, chatRepo)
+		mohongRepo = mohong.NewRepository(deps.DB)
 		mohongService = mohong.NewService(mohongRepo)
 		mohongHandler = mohong.NewHandler(mohongService)
 	}