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/stores/payment.ts

@@ -0,0 +1,101 @@
+/**
+ * Payment Store
+ * Manages payment configuration, current order state, and subscription plans
+ */
+
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import { paymentAPI } from '@/api/payment'
+import type { PaymentConfig, PaymentOrder, SubscriptionPlan, CreateOrderRequest } from '@/types/payment'
+
+export const usePaymentStore = defineStore('payment', () => {
+  // ==================== State ====================
+
+  /** Payment configuration from backend */
+  const config = ref<PaymentConfig | null>(null)
+  /** Currently active order (for payment flow) */
+  const currentOrder = ref<PaymentOrder | null>(null)
+  /** Available subscription plans */
+  const plans = ref<SubscriptionPlan[]>([])
+
+  const configLoading = ref(false)
+  const configLoaded = ref(false)
+
+  // ==================== Actions ====================
+
+  /** Fetch payment configuration */
+  async function fetchConfig(force = false): Promise<PaymentConfig | null> {
+    if (configLoaded.value && !force) return config.value
+    if (configLoading.value) return config.value
+
+    configLoading.value = true
+    try {
+      const response = await paymentAPI.getConfig()
+      config.value = response.data
+      configLoaded.value = true
+      return config.value
+    } catch (error: unknown) {
+      console.error('[payment] Failed to fetch config:', error)
+      return null
+    } finally {
+      configLoading.value = false
+    }
+  }
+
+  /** Fetch available subscription plans */
+  async function fetchPlans(): Promise<SubscriptionPlan[]> {
+    try {
+      const response = await paymentAPI.getPlans()
+      // Backend returns features as newline-separated string; parse to array
+      plans.value = (response.data || []).map((p: Omit<SubscriptionPlan, 'features'> & { features: string | string[] }) => ({
+        ...p,
+        features: typeof p.features === 'string'
+          ? p.features.split('\n').map((f: string) => f.trim()).filter(Boolean)
+          : (p.features || []),
+      }))
+      return plans.value
+    } catch (error: unknown) {
+      console.error('[payment] Failed to fetch plans:', error)
+      return []
+    }
+  }
+
+  /** Create a new order and set it as current */
+  async function createOrder(params: CreateOrderRequest) {
+    const response = await paymentAPI.createOrder(params)
+    return response.data
+  }
+
+  /** Poll order status by ID */
+  async function pollOrderStatus(orderId: number): Promise<PaymentOrder | null> {
+    try {
+      const response = await paymentAPI.getOrder(orderId)
+      const order = response.data
+      if (currentOrder.value?.id === orderId) {
+        currentOrder.value = order
+      }
+      return order
+    } catch (error: unknown) {
+      console.error('[payment] Failed to poll order status:', error)
+      return null
+    }
+  }
+
+  /** Clear current order state */
+  function clearCurrentOrder() {
+    currentOrder.value = null
+  }
+
+  return {
+    config,
+    currentOrder,
+    plans,
+    configLoading,
+    configLoaded,
+    fetchConfig,
+    fetchPlans,
+    createOrder,
+    pollOrderStatus,
+    clearCurrentOrder
+  }
+})