修复(#75): 按数量精确匹配套餐列表促销

- loadSubscribePromoMap 返回 map[subscribeID]map[quantity]*SubscribePromo 二级映射
- 候选查询按 subscribe_id、quantity、priority DESC 排序，一次取出所有 quantity 档位
- 下单路径 QueryEligibleRules 将 quantity 条件移至 JOIN，精确匹配
- 永久订阅(expire_time=0)用 CASE WHEN 排序兜底，不再误判为回归促销
- 新增 promoEligibility、model、subscribe promo 单元测试

Co-authored-by: multica-agent <github@multica.ai>

internal/logic/public/order/promoPricing_test.go

@@ -32,19 +32,20 @@ func (m *fakePromoModel) InsertUsage(context.Context, *promo.Usage, ...*gorm.DB)
 }
 
 func TestCalculatePurchasePricePromoSkipsPercentDiscount(t *testing.T) {
-	svcCtx := &svc.ServiceContext{
-		DB: &gorm.DB{},
-		PromoModel: &fakePromoModel{rules: []*promo.RuleWithPrice{
-			{
-				Rule: promo.Rule{
-					Id:      9,
-					Name:    "campaign",
-					Type:    promo.RuleTypeCampaign,
-					Enabled: true,
-				},
-				PromoPrice: 600,
+	model := &fakePromoModel{rules: []*promo.RuleWithPrice{
+		{
+			Rule: promo.Rule{
+				Id:      9,
+				Name:    "campaign",
+				Type:    promo.RuleTypeCampaign,
+				Enabled: true,
 			},
-		}},
+			PromoPrice: 600,
+		},
+	}}
+	svcCtx := &svc.ServiceContext{
+		DB:         &gorm.DB{},
+		PromoModel: model,
 	}
 
 	result, err := calculatePurchasePrice(
@@ -77,22 +78,26 @@ func TestCalculatePurchasePricePromoSkipsPercentDiscount(t *testing.T) {
 	if result.PromoDiscount != 1200 {
 		t.Fatalf("PromoDiscount = %d, want 1200", result.PromoDiscount)
 	}
+	if model.lastQuantity != 3 {
+		t.Fatalf("promo query quantity = %d, want 3", model.lastQuantity)
+	}
 }
 
 func TestCalculatePurchasePriceIgnoresInvalidPromoPrice(t *testing.T) {
-	svcCtx := &svc.ServiceContext{
-		DB: &gorm.DB{},
-		PromoModel: &fakePromoModel{rules: []*promo.RuleWithPrice{
-			{
-				Rule: promo.Rule{
-					Id:      10,
-					Name:    "invalid campaign",
-					Type:    promo.RuleTypeCampaign,
-					Enabled: true,
-				},
-				PromoPrice: 1000,
+	model := &fakePromoModel{rules: []*promo.RuleWithPrice{
+		{
+			Rule: promo.Rule{
+				Id:      10,
+				Name:    "invalid campaign",
+				Type:    promo.RuleTypeCampaign,
+				Enabled: true,
 			},
-		}},
+			PromoPrice: 1000,
+		},
+	}}
+	svcCtx := &svc.ServiceContext{
+		DB:         &gorm.DB{},
+		PromoModel: model,
 	}
 
 	result, err := calculatePurchasePrice(