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/promoPricing_test.go

@@ -11,21 +11,30 @@ import (
 )
 
 type fakePromoModel struct {
-	rules []*promo.RuleWithPrice
+	rules            []*promo.RuleWithPrice
+	lastSubscribeID  int64
+	lastQuantity     int64
+	requireQuantity  int64
+	quantityMismatch []*promo.RuleWithPrice
 }
 
-func (m fakePromoModel) QueryEligibleRules(context.Context, int64) ([]*promo.RuleWithPrice, error) {
+func (m *fakePromoModel) QueryEligibleRules(_ context.Context, subscribeID int64, quantity int64) ([]*promo.RuleWithPrice, error) {
+	m.lastSubscribeID = subscribeID
+	m.lastQuantity = quantity
+	if m.requireQuantity > 0 && quantity != m.requireQuantity {
+		return m.quantityMismatch, nil
+	}
 	return m.rules, nil
 }
 
-func (m fakePromoModel) InsertUsage(context.Context, *promo.Usage, ...*gorm.DB) error {
+func (m *fakePromoModel) InsertUsage(context.Context, *promo.Usage, ...*gorm.DB) error {
 	return nil
 }
 
 func TestCalculatePurchasePricePromoSkipsPercentDiscount(t *testing.T) {
 	svcCtx := &svc.ServiceContext{
 		DB: &gorm.DB{},
-		PromoModel: fakePromoModel{rules: []*promo.RuleWithPrice{
+		PromoModel: &fakePromoModel{rules: []*promo.RuleWithPrice{
 			{
 				Rule: promo.Rule{
 					Id:      9,
@@ -73,7 +82,7 @@ func TestCalculatePurchasePricePromoSkipsPercentDiscount(t *testing.T) {
 func TestCalculatePurchasePriceIgnoresInvalidPromoPrice(t *testing.T) {
 	svcCtx := &svc.ServiceContext{
 		DB: &gorm.DB{},
-		PromoModel: fakePromoModel{rules: []*promo.RuleWithPrice{
+		PromoModel: &fakePromoModel{rules: []*promo.RuleWithPrice{
 			{
 				Rule: promo.Rule{
 					Id:      10,
@@ -111,3 +120,52 @@ func TestCalculatePurchasePriceIgnoresInvalidPromoPrice(t *testing.T) {
 		t.Fatalf("promo fields = (%d, %d), want (0, 0)", result.PromoRuleId, result.PromoDiscount)
 	}
 }
+
+func TestCalculatePurchasePricePassesQuantityToPromoEvaluation(t *testing.T) {
+	promoModel := &fakePromoModel{
+		requireQuantity: 6,
+		rules: []*promo.RuleWithPrice{
+			{
+				Rule: promo.Rule{
+					Id:      11,
+					Name:    "quantity campaign",
+					Type:    promo.RuleTypeCampaign,
+					Enabled: true,
+				},
+				PromoPrice: 500,
+			},
+		},
+	}
+	svcCtx := &svc.ServiceContext{
+		DB:         &gorm.DB{},
+		PromoModel: promoModel,
+	}
+
+	result, err := calculatePurchasePrice(
+		context.Background(),
+		svcCtx,
+		1,
+		2,
+		1000,
+		6,
+		[]types.SubscribeDiscount{{Quantity: 6, Discount: 80}},
+		true,
+		true,
+	)
+	if err != nil {
+		t.Fatalf("calculatePurchasePrice returned error: %v", err)
+	}
+
+	if promoModel.lastSubscribeID != 2 {
+		t.Fatalf("lastSubscribeID = %d, want 2", promoModel.lastSubscribeID)
+	}
+	if promoModel.lastQuantity != 6 {
+		t.Fatalf("lastQuantity = %d, want 6", promoModel.lastQuantity)
+	}
+	if result.PayableBase != 3000 {
+		t.Fatalf("PayableBase = %d, want 3000", result.PayableBase)
+	}
+	if result.PromoRuleId != 11 {
+		t.Fatalf("PromoRuleId = %d, want 11", result.PromoRuleId)
+	}
+}