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/order/preCreateOrderLogic.go

@@ -47,13 +47,18 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		l.Debugf("[PreCreateOrder] Quantity is less than or equal to 0, setting to 1")
 		req.Quantity = 1
 	}
+	entitlement, entErr := commonLogic.ResolveEntitlementUser(l.ctx, l.svcCtx.DB, u.Id)
+	if entErr != nil {
+		return nil, entErr
+	}
 
 	targetSubscribeID := req.SubscribeId
+	orderType := uint8(1)
 	isSingleModeRenewal := false
 	decision, routeErr := commonLogic.ResolvePurchaseRoute(
 		l.ctx,
 		l.svcCtx.Config.Subscribe.SingleModel,
-		u.Id,
+		entitlement.EffectiveUserID,
 		req.SubscribeId,
 		l.svcCtx.UserModel.FindSingleModeAnchorSubscribe,
 	)
@@ -68,15 +73,44 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		targetSubscribeID = decision.ResolvedSubscribeID
 		isSingleModeRenewal = decision.Route == commonLogic.PurchaseRoutePurchaseToRenewal
 		if isSingleModeRenewal && decision.Anchor != nil {
+			orderType = 2
 			l.Infow("[PreCreateOrder] single mode purchase routed to renewal preview",
 				logger.Field("mode", "single"),
 				logger.Field("route", "purchase_to_renewal"),
 				logger.Field("anchor_user_subscribe_id", decision.Anchor.Id),
 				logger.Field("user_id", u.Id),
+				logger.Field("effective_user_id", entitlement.EffectiveUserID),
 			)
 		}
 	}
 
+	// Keep promo eligibility preview aligned with Purchase: an existing paid subscription
+	// routes the request to renewal semantics, where first-purchase promos are disabled.
+	if !l.svcCtx.Config.Subscribe.SingleModel && orderType == 1 {
+		var existSub user.Subscribe
+		if e := l.svcCtx.DB.WithContext(l.ctx).
+			Model(&user.Subscribe{}).
+			Where("user_id = ? AND token != '' AND (order_id > 0 OR token LIKE 'iap:%')", entitlement.EffectiveUserID).
+			Order("expire_time DESC").
+			Order("updated_at DESC").
+			Order("id DESC").
+			First(&existSub).Error; e == nil && existSub.Id > 0 && existSub.Token != "" {
+			orderType = 2
+			l.Infow("[PreCreateOrder] purchase preview routed to renewal because an existing subscription was found",
+				logger.Field("route_mode", "global_single_subscription"),
+				logger.Field("route", "purchase_to_existing_subscription"),
+				logger.Field("existing_subscribe_id", existSub.Id),
+				logger.Field("existing_status", existSub.Status),
+				logger.Field("user_id", u.Id),
+				logger.Field("effective_user_id", entitlement.EffectiveUserID),
+				logger.Field("resolved_subscribe_id", targetSubscribeID),
+			)
+		} else if e != nil && !errors.Is(e, gorm.ErrRecordNotFound) {
+			l.Errorw("[PreCreateOrder] Database query error", logger.Field("error", e.Error()), logger.Field("user_id", u.Id))
+			return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseQueryError), "find existing subscription error: %v", e.Error())
+		}
+	}
+
 	// find subscribe plan
 	sub, err := l.svcCtx.SubscribeModel.FindOne(l.ctx, targetSubscribeID)
 	if err != nil {
@@ -86,7 +120,7 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 
 	// check subscribe plan quota limit for new purchase flow only
 	if !isSingleModeRenewal && sub.Quota > 0 {
-		userSub, err := l.svcCtx.UserModel.QueryUserSubscribe(l.ctx, u.Id)
+		userSub, err := l.svcCtx.UserModel.QueryUserSubscribe(l.ctx, entitlement.EffectiveUserID)
 		if err != nil {
 			l.Errorw("[PreCreateOrder] Database query error", logger.Field("error", err.Error()), logger.Field("user_id", u.Id))
 			return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseQueryError), "find user subscription error: %v", err.Error())
@@ -102,7 +136,7 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		}
 	}
 
-	newUserDiscount, err := resolveNewUserDiscountEligibility(l.ctx, l.svcCtx.DB, u.Id, targetSubscribeID, req.Quantity, sub.Discount)
+	newUserDiscount, err := resolveNewUserDiscountEligibility(l.ctx, l.svcCtx.DB, entitlement.EffectiveUserID, targetSubscribeID, req.Quantity, sub.Discount)
 	if err != nil {
 		l.Errorw("[PreCreateOrder] Database query error resolving new user eligibility",
 			logger.Field("error", err.Error()),
@@ -117,13 +151,13 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 	priceResult, err := calculatePurchasePrice(
 		l.ctx,
 		l.svcCtx,
-		u.Id,
+		entitlement.EffectiveUserID,
 		targetSubscribeID,
 		sub.UnitPrice,
 		req.Quantity,
 		newUserDiscount.Discounts,
 		newUserDiscount.EligibleForDiscount,
-		!isSingleModeRenewal,
+		orderType == 1,
 	)
 	if err != nil {
 		l.Errorw("[PreCreateOrder] Promo price calculation error",