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/logic/common/promoEligibility.go

@@ -27,13 +27,13 @@ type promoRuleParams struct {
 	InactiveMonths int `json:"inactive_months"`
 }
 
-func EvaluatePromo(ctx context.Context, svcCtx *svc.ServiceContext, userID int64, subscribeID int64) (*PromoResult, error) {
+func EvaluatePromo(ctx context.Context, svcCtx *svc.ServiceContext, userID int64, subscribeID int64, quantity int64) (*PromoResult, error) {
 	result := &PromoResult{}
-	if svcCtx == nil || svcCtx.PromoModel == nil || svcCtx.DB == nil || userID <= 0 || subscribeID <= 0 {
+	if svcCtx == nil || svcCtx.PromoModel == nil || svcCtx.DB == nil || userID <= 0 || subscribeID <= 0 || quantity <= 0 {
 		return result, nil
 	}
 
-	rules, err := svcCtx.PromoModel.QueryEligibleRules(ctx, subscribeID)
+	rules, err := svcCtx.PromoModel.QueryEligibleRules(ctx, subscribeID, quantity)
 	if err != nil {
 		return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseQueryError), "query promo rules failed: %v", err.Error())
 	}