feat: 实现注册优惠码功能

- 支持创建/编辑/删除优惠码，设置赠送金额和使用限制
  - 注册页面实时验证优惠码并显示赠送金额
  - 支持 URL 参数自动填充 (?promo=CODE)
  - 添加优惠码验证接口速率限制
  - 使用数据库行锁防止并发超限
  - 新增后台优惠码管理页面，支持复制注册链接

frontend/src/api/admin/index.ts

@@ -9,6 +9,7 @@ import groupsAPI from './groups'
 import accountsAPI from './accounts'
 import proxiesAPI from './proxies'
 import redeemAPI from './redeem'
+import promoAPI from './promo'
 import settingsAPI from './settings'
 import systemAPI from './system'
 import subscriptionsAPI from './subscriptions'
@@ -27,6 +28,7 @@ export const adminAPI = {
   accounts: accountsAPI,
   proxies: proxiesAPI,
   redeem: redeemAPI,
+  promo: promoAPI,
   settings: settingsAPI,
   system: systemAPI,
   subscriptions: subscriptionsAPI,
@@ -43,6 +45,7 @@ export {
   accountsAPI,
   proxiesAPI,
   redeemAPI,
+  promoAPI,
   settingsAPI,
   systemAPI,
   subscriptionsAPI,