feat: 实现站内USDT收银台支付流程

- 后端: payment.go 支付处理(创建订单/回调通知/状态查询/签名算法)
- 后端: 新增公开支付通道API(PublicList,不暴露config)
- 前端: Checkout.vue 收银台页面(倒计时/二维码/支付轮询)
- 前端: Cart.vue 对接动态支付通道,选择bepusdt跳转收银台
- 前端: API添加paymentChannelApi和orderApi支付方法
- 路由: 注册checkout路由和支付相关API

frontend/src/api/index.ts

@@ -1,5 +1,9 @@
 import api from '../utils/request'
 
+export const paymentChannelApi = {
+  list: () => api.get('/payment-channels'),
+}
+
 export const uploadApi = {
   uploadImage: (file: File) => {
     const formData = new FormData()
@@ -74,6 +78,8 @@ export const orderApi = {
   refund: (id: number, data: any) => api.post(`/orders/${id}/refund`, data),
   cancel: (id: number) => api.put(`/orders/${id}/cancel`),
   confirmReceipt: (id: number) => api.put(`/orders/${id}/confirm-receipt`),
+  createPayment: (id: number) => api.post(`/orders/${id}/pay`),
+  checkPaymentStatus: (id: number) => api.get(`/orders/${id}/payment-status`),
 }
 
 export const articleApi = {