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/model/promo/model.go

@@ -27,18 +27,22 @@ func NewModel(db *gorm.DB, _ *redis.Client) Model {
 
 func (m *defaultPromoModel) QueryEligibleRules(ctx context.Context, subscribeId int64, quantity int64) ([]*RuleWithPrice, error) {
 	var list []*RuleWithPrice
-	err := m.db.WithContext(ctx).
-		Table("promo_rule AS pr").
-		Select("pr.*, sp.promo_price").
-		Joins("JOIN subscribe_promo AS sp ON sp.promo_rule_id = pr.id").
-		Where("sp.subscribe_id = ? AND sp.quantity = ? AND sp.promo_price > 0 AND pr.enabled = ?", subscribeId, quantity, true).
-		Where("pr.deleted_at IS NULL").
-		Order("pr.priority DESC").
-		Order("pr.id ASC").
+	err := m.eligibleRulesQuery(ctx, subscribeId, quantity).
 		Find(&list).Error
 	return list, err
 }
 
+func (m *defaultPromoModel) eligibleRulesQuery(ctx context.Context, subscribeId int64, quantity int64) *gorm.DB {
+	return m.db.WithContext(ctx).
+		Table("promo_rule AS pr").
+		Select("pr.*, sp.promo_price").
+		Joins("JOIN subscribe_promo AS sp ON sp.promo_rule_id = pr.id AND sp.quantity = ?", quantity).
+		Where("sp.subscribe_id = ? AND sp.promo_price > 0 AND pr.enabled = ?", subscribeId, true).
+		Where("pr.deleted_at IS NULL").
+		Order("pr.priority DESC").
+		Order("pr.id ASC")
+}
+
 func (m *defaultPromoModel) InsertUsage(ctx context.Context, data *Usage, tx ...*gorm.DB) error {
 	db := m.db.WithContext(ctx)
 	if len(tx) > 0 {