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/user/where.go

@@ -871,6 +871,29 @@ func HasAttributeValuesWith(preds ...predicate.UserAttributeValue) predicate.Use
 	})
 }
 
+// HasPromoCodeUsages applies the HasEdge predicate on the "promo_code_usages" edge.
+func HasPromoCodeUsages() predicate.User {
+	return predicate.User(func(s *sql.Selector) {
+		step := sqlgraph.NewStep(
+			sqlgraph.From(Table, FieldID),
+			sqlgraph.Edge(sqlgraph.O2M, false, PromoCodeUsagesTable, PromoCodeUsagesColumn),
+		)
+		sqlgraph.HasNeighbors(s, step)
+	})
+}
+
+// HasPromoCodeUsagesWith applies the HasEdge predicate on the "promo_code_usages" edge with a given conditions (other predicates).
+func HasPromoCodeUsagesWith(preds ...predicate.PromoCodeUsage) predicate.User {
+	return predicate.User(func(s *sql.Selector) {
+		step := newPromoCodeUsagesStep()
+		sqlgraph.HasNeighborsWith(s, step, func(s *sql.Selector) {
+			for _, p := range preds {
+				p(s)
+			}
+		})
+	})
+}
+
 // HasUserAllowedGroups applies the HasEdge predicate on the "user_allowed_groups" edge.
 func HasUserAllowedGroups() predicate.User {
 	return predicate.User(func(s *sql.Selector) {