feat(wallet): 钱包系统替代配额系统，新增支付与退款机制

- 新增钱包余额管理、充值、扣费、退款完整流程
- 新增支付网关抽象层（支持手动/支付宝/微信）
- 用量计费从配额系统迁移到钱包余额扣费
- 新增管理员钱包管理与支付订单管理页面
- 新增用户钱包中心页面
- 移除独立 Key 锁定机制，统一由钱包余额控制
- 新增相关 API 路由、序列化器与数据库迁移
- 新增钱包、支付、退款相关测试

frontend/src/mocks/handler.ts

@@ -588,8 +588,22 @@ const mockHandlers: Record<string, (config: AxiosRequestConfig) => Promise<Axios
       total_cost: Number((totalCost * 20).toFixed(2)),
       total_actual_cost: Number((totalActualCost * 20).toFixed(2)),
       avg_response_time: Number(avgResponseTime.toFixed(2)) || 1.23,
-      quota_usd: 100,
-      used_usd: Number((totalCost * 20).toFixed(2)),
+      billing: {
+        id: 'wallet-demo-user',
+        balance: Number((100 - totalCost * 20).toFixed(2)),
+        recharge_balance: Number((100 - totalCost * 20).toFixed(2)),
+        gift_balance: 0,
+        refundable_balance: Number((100 - totalCost * 20).toFixed(2)),
+        currency: 'USD',
+        status: 'active',
+        limit_mode: 'finite',
+        unlimited: false,
+        total_recharged: 100,
+        total_consumed: Number((totalCost * 20).toFixed(2)),
+        total_refunded: 0,
+        total_adjusted: 0,
+        updated_at: new Date().toISOString(),
+      },
       activity_heatmap: heatmap,
       summary_by_model: Array.from(modelStats.entries()).map(([model, stats]) => ({
         model,
@@ -726,10 +740,8 @@ const mockHandlers: Record<string, (config: AxiosRequestConfig) => Promise<Axios
       username: body.username,
       email: body.email,
       role: body.role || 'user',
+      unlimited: Boolean(body.unlimited),
       is_active: true,
-      quota_usd: body.quota_usd || null,
-      used_usd: 0,
-      total_usd: 0,
       allowed_providers: null,
       allowed_api_formats: null,
       allowed_models: null,
@@ -757,8 +769,6 @@ const mockHandlers: Record<string, (config: AxiosRequestConfig) => Promise<Axios
       key_display: 'sk-sa...demo',
       is_active: true,
       is_standalone: true,
-      balance_used_usd: 0,
-      current_balance_usd: body.initial_balance_usd || 100,
       total_requests: 0,
       created_at: new Date().toISOString()
     }