修复(#85): 促销系统 quantity 设计修正补丁（跨任务统一修复）

- subscribe_promo 表加 quantity 字段，BIGINT NOT NULL DEFAULT 1
- EvaluatePromo 加 quantity 参数，按 subscribeID + quantity 精确匹配
- Promo 从 Subscribe 顶层移到 SubscribeDiscount
- 查询加 quantity，返回 map[subscribeID][quantity] 二级映射
- recordPromoUsage 错误向上传播，不再静默吞掉
- preCreate 和 purchase 的 allowPromo 判定统一为 orderType==1
- 迁移脚本增加幂等处理（guarded DROP/ADD/MODIFY）

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

internal/model/promo/promo.go

@@ -33,6 +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"`
 	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"`