feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

frontend/src/api/payment.ts

@@ -0,0 +1,79 @@
+/**
+ * User Payment API endpoints
+ * Handles payment operations for regular users
+ */
+
+import { apiClient } from './client'
+import type {
+  PaymentConfig,
+  SubscriptionPlan,
+  PaymentChannel,
+  MethodLimitsResponse,
+  CheckoutInfoResponse,
+  CreateOrderRequest,
+  CreateOrderResult,
+  PaymentOrder
+} from '@/types/payment'
+import type { BasePaginationResponse } from '@/types'
+
+export const paymentAPI = {
+  /** Get payment configuration (enabled types, limits, etc.) */
+  getConfig() {
+    return apiClient.get<PaymentConfig>('/payment/config')
+  },
+
+  /** Get available subscription plans */
+  getPlans() {
+    return apiClient.get<SubscriptionPlan[]>('/payment/plans')
+  },
+
+  /** Get available payment channels */
+  getChannels() {
+    return apiClient.get<PaymentChannel[]>('/payment/channels')
+  },
+
+  /** Get all checkout page data in a single call */
+  getCheckoutInfo() {
+    return apiClient.get<CheckoutInfoResponse>('/payment/checkout-info')
+  },
+
+  /** Get payment method limits and fee rates */
+  getLimits() {
+    return apiClient.get<MethodLimitsResponse>('/payment/limits')
+  },
+
+  /** Create a new payment order */
+  createOrder(data: CreateOrderRequest) {
+    return apiClient.post<CreateOrderResult>('/payment/orders', data)
+  },
+
+  /** Get current user's orders */
+  getMyOrders(params?: { page?: number; page_size?: number; status?: string }) {
+    return apiClient.get<BasePaginationResponse<PaymentOrder>>('/payment/orders/my', { params })
+  },
+
+  /** Get a specific order by ID */
+  getOrder(id: number) {
+    return apiClient.get<PaymentOrder>(`/payment/orders/${id}`)
+  },
+
+  /** Cancel a pending order */
+  cancelOrder(id: number) {
+    return apiClient.post(`/payment/orders/${id}/cancel`)
+  },
+
+  /** Verify order payment status with upstream provider */
+  verifyOrder(outTradeNo: string) {
+    return apiClient.post<PaymentOrder>('/payment/orders/verify', { out_trade_no: outTradeNo })
+  },
+
+  /** Verify order payment status without auth (public endpoint for result page) */
+  verifyOrderPublic(outTradeNo: string) {
+    return apiClient.post<PaymentOrder>('/payment/public/orders/verify', { out_trade_no: outTradeNo })
+  },
+
+  /** Request a refund for a completed order */
+  requestRefund(id: number, data: { reason: string }) {
+    return apiClient.post(`/payment/orders/${id}/refund-request`, data)
+  }
+}