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/ent/tx.go

@@ -22,6 +22,10 @@ type Tx struct {
 	AccountGroup *AccountGroupClient
 	// Group is the client for interacting with the Group builders.
 	Group *GroupClient
+	// PromoCode is the client for interacting with the PromoCode builders.
+	PromoCode *PromoCodeClient
+	// PromoCodeUsage is the client for interacting with the PromoCodeUsage builders.
+	PromoCodeUsage *PromoCodeUsageClient
 	// Proxy is the client for interacting with the Proxy builders.
 	Proxy *ProxyClient
 	// RedeemCode is the client for interacting with the RedeemCode builders.
@@ -175,6 +179,8 @@ func (tx *Tx) init() {
 	tx.Account = NewAccountClient(tx.config)
 	tx.AccountGroup = NewAccountGroupClient(tx.config)
 	tx.Group = NewGroupClient(tx.config)
+	tx.PromoCode = NewPromoCodeClient(tx.config)
+	tx.PromoCodeUsage = NewPromoCodeUsageClient(tx.config)
 	tx.Proxy = NewProxyClient(tx.config)
 	tx.RedeemCode = NewRedeemCodeClient(tx.config)
 	tx.Setting = NewSettingClient(tx.config)