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

backend/internal/handler/wire.go

@@ -19,6 +19,7 @@ func ProvideAdminHandlers(
 	antigravityOAuthHandler *admin.AntigravityOAuthHandler,
 	proxyHandler *admin.ProxyHandler,
 	redeemHandler *admin.RedeemHandler,
+	promoHandler *admin.PromoHandler,
 	settingHandler *admin.SettingHandler,
 	systemHandler *admin.SystemHandler,
 	subscriptionHandler *admin.SubscriptionHandler,
@@ -36,6 +37,7 @@ func ProvideAdminHandlers(
 		AntigravityOAuth: antigravityOAuthHandler,
 		Proxy:            proxyHandler,
 		Redeem:           redeemHandler,
+		Promo:            promoHandler,
 		Setting:          settingHandler,
 		System:           systemHandler,
 		Subscription:     subscriptionHandler,
@@ -105,6 +107,7 @@ var ProviderSet = wire.NewSet(
 	admin.NewAntigravityOAuthHandler,
 	admin.NewProxyHandler,
 	admin.NewRedeemHandler,
+	admin.NewPromoHandler,
 	admin.NewSettingHandler,
 	ProvideSystemHandler,
 	admin.NewSubscriptionHandler,