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/public/subscribe/promo.go

@@ -57,6 +57,12 @@ func loadSubscribePromoMap(ctx context.Context, svcCtx *svc.ServiceContext, subs
 	evaluator := promoEligibilityEvaluator{ctx: ctx, db: svcCtx.DB, userInfo: userInfo}
 	now := time.Now()
 	for _, candidate := range candidates {
+		if candidate.Quantity <= 0 {
+			continue
+		}
+		if result[candidate.SubscribeId] == nil {
+			result[candidate.SubscribeId] = make(map[int64]*types.SubscribePromo)
+		}
 		if _, exists := result[candidate.SubscribeId][candidate.Quantity]; exists {
 			continue
 		}
@@ -70,9 +76,6 @@ func loadSubscribePromoMap(ctx context.Context, svcCtx *svc.ServiceContext, subs
 		if !ok {
 			continue
 		}
-		if _, exists := result[candidate.SubscribeId]; !exists {
-			result[candidate.SubscribeId] = make(map[int64]*types.SubscribePromo)
-		}
 		result[candidate.SubscribeId][candidate.Quantity] = &types.SubscribePromo{
 			RuleName:   candidate.RuleName,
 			RuleType:   candidate.RuleType,