补充 VP 模块注册 + 前端 API 封装

- app.module.ts: 注册 VirtualPaymentModule
- config.ts + api.ts: 添加虚拟支付 API 端点
- useGravityPurchase.ts: VP 购买逻辑
- interview.vue / user.vue: 适配 VP 调用
- admin.controller.ts: 管理端虚拟支付相关调整
- payment.controller.spec.ts: 测试适配

backend/src/modules/admin/admin.controller.ts

@@ -428,8 +428,8 @@ const DEFAULT_CONFIG = {
 }
 
 const DEFAULT_PRICING = {
-  interview: { pricePerSession: 500, creditsPerPurchase: 1 },
-  resumeOptimize: { freeLimit: 3, pricePerOptimize: 300, creditsPerPurchase: 1 },
+  interview: { pricePerSession: 600, creditsPerPurchase: 1 },
+  resumeOptimize: { freeLimit: 3, pricePerOptimize: 400, creditsPerPurchase: 1 },
   resumeDownload: { pricePerDownload: 200, creditsPerPurchase: 1 },
   plans: {
     growth: {