修复(#79): 促销管理后台API + 审查问题修复

- 实现促销管理接口：规则CRUD、优惠价配置、使用记录查询
- 修复 UpsertPrices 新增记录时提前 return 的问题
- DeleteRule/DeletePrice 不存在记录返回 code=404
- SetPromoPriceRequest.items 空数组校验
- 分页参数限制 page>0、1<=size<=200
- 下单侧促销价格按数量档总价口径计算（含 #87 修复）

Co-authored-by: multica-agent <github@multica.ai>

internal/model/promo/promo.go

@@ -33,7 +33,7 @@ func (Rule) TableName() string {
 type SubscribePromo struct {
 	Id          int64     `gorm:"primaryKey"`
 	SubscribeId int64     `gorm:"type:bigint unsigned;not null;comment:Subscribe ID"`
-	Quantity    int64     `gorm:"type:bigint;not null;default:1;comment:Quantity"`
+	Quantity    int64     `gorm:"type:int;not null;default:0;comment:购买数量"`
 	PromoRuleId int64     `gorm:"type:bigint unsigned;not null;comment:Promo Rule ID"`
 	PromoPrice  int64     `gorm:"type:bigint;not null;default:0;comment:Promo Price"`
 	CreatedAt   time.Time `gorm:"<-:create;comment:Create Time"`