feat: 改造支付回调 Handler，支持富友回调和多订单类型按前缀分发

- payment.go: WechatPayCallback 改造为按订单号前缀分发（ORD→套餐订单、CRCH→资产充值、ARCH→代理充值）；新增 FuiouPayCallback（GBK→UTF-8+XML解析+验签+分发）；修复 RechargeOrderPrefix 废弃引用
- order.go: 注册 POST /api/callback/fuiou-pay 路由（无需认证）

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/routes/order.go

@@ -121,4 +121,12 @@ func registerPaymentCallbackRoutes(router fiber.Router, handler *callback.Paymen
 		Output:  nil,
 		Auth:    false,
 	})
+
+	Register(router, doc, basePath, "POST", "/fuiou-pay", handler.FuiouPayCallback, RouteSpec{
+		Summary: "富友支付回调",
+		Tags:    []string{"支付回调"},
+		Input:   nil,
+		Output:  nil,
+		Auth:    false,
+	})
 }