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/model.go

@@ -13,7 +13,7 @@ type RuleWithPrice struct {
 }
 
 type Model interface {
-	QueryEligibleRules(ctx context.Context, subscribeId int64) ([]*RuleWithPrice, error)
+	QueryEligibleRules(ctx context.Context, subscribeId int64, quantity int64) ([]*RuleWithPrice, error)
 	InsertUsage(ctx context.Context, data *Usage, tx ...*gorm.DB) error
 }
 
@@ -25,13 +25,13 @@ func NewModel(db *gorm.DB, _ *redis.Client) Model {
 	return &defaultPromoModel{db: db}
 }
 
-func (m *defaultPromoModel) QueryEligibleRules(ctx context.Context, subscribeId int64) ([]*RuleWithPrice, error) {
+func (m *defaultPromoModel) QueryEligibleRules(ctx context.Context, subscribeId int64, quantity int64) ([]*RuleWithPrice, error) {
 	var list []*RuleWithPrice
 	err := m.db.WithContext(ctx).
 		Table("promo_rule AS pr").
 		Select("pr.*, sp.promo_price").
 		Joins("JOIN subscribe_promo AS sp ON sp.promo_rule_id = pr.id").
-		Where("sp.subscribe_id = ? AND sp.promo_price > 0 AND pr.enabled = ?", subscribeId, true).
+		Where("sp.subscribe_id = ? AND sp.quantity = ? AND sp.promo_price > 0 AND pr.enabled = ?", subscribeId, quantity, true).
 		Where("pr.deleted_at IS NULL").
 		Order("pr.priority DESC").
 		Order("pr.id ASC").

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"`