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/hook/hook.go

@@ -57,6 +57,30 @@ func (f GroupFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error
 	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.GroupMutation", m)
 }
 
+// The PromoCodeFunc type is an adapter to allow the use of ordinary
+// function as PromoCode mutator.
+type PromoCodeFunc func(context.Context, *ent.PromoCodeMutation) (ent.Value, error)
+
+// Mutate calls f(ctx, m).
+func (f PromoCodeFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
+	if mv, ok := m.(*ent.PromoCodeMutation); ok {
+		return f(ctx, mv)
+	}
+	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.PromoCodeMutation", m)
+}
+
+// The PromoCodeUsageFunc type is an adapter to allow the use of ordinary
+// function as PromoCodeUsage mutator.
+type PromoCodeUsageFunc func(context.Context, *ent.PromoCodeUsageMutation) (ent.Value, error)
+
+// Mutate calls f(ctx, m).
+func (f PromoCodeUsageFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
+	if mv, ok := m.(*ent.PromoCodeUsageMutation); ok {
+		return f(ctx, mv)
+	}
+	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.PromoCodeUsageMutation", m)
+}
+
 // The ProxyFunc type is an adapter to allow the use of ordinary
 // function as Proxy mutator.
 type ProxyFunc func(context.Context, *ent.ProxyMutation) (ent.Value, error)