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/subscribe/promo.go

@@ -15,6 +15,7 @@ import (
 	"github.com/perfect-panel/server/pkg/xerr"
 	"github.com/pkg/errors"
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 )
 
 const (
@@ -89,7 +90,16 @@ func loadSubscribePromoMap(ctx context.Context, svcCtx *svc.ServiceContext, subs
 
 func querySubscribePromoCandidates(ctx context.Context, svcCtx *svc.ServiceContext, subscribeIDs []int64, loggedIn bool) ([]subscribePromoCandidate, error) {
 	var candidates []subscribePromoCandidate
-	query := svcCtx.DB.WithContext(ctx).
+	err := subscribePromoCandidatesQuery(ctx, svcCtx.DB, subscribeIDs, loggedIn).
+		Scan(&candidates).Error
+	if err != nil {
+		return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseQueryError), "query subscribe promo candidates failed: %v", err)
+	}
+	return candidates, nil
+}
+
+func subscribePromoCandidatesQuery(ctx context.Context, db *gorm.DB, subscribeIDs []int64, loggedIn bool) *gorm.DB {
+	query := db.WithContext(ctx).
 		Table("subscribe_promo AS sp").
 		Select("sp.subscribe_id, sp.quantity, sp.promo_price, pr.name AS rule_name, pr.type AS rule_type, pr.params, pr.start_time, pr.end_time").
 		Joins("JOIN promo_rule AS pr ON pr.id = sp.promo_rule_id AND pr.deleted_at IS NULL").
@@ -97,16 +107,11 @@ func querySubscribePromoCandidates(ctx context.Context, svcCtx *svc.ServiceConte
 	if !loggedIn {
 		query = query.Where("pr.type = ?", promoRuleTypeCampaign)
 	}
-	err := query.
+	return query.
 		Order("sp.subscribe_id ASC").
 		Order("sp.quantity ASC").
 		Order("pr.priority DESC").
-		Order("pr.id ASC").
-		Scan(&candidates).Error
-	if err != nil {
-		return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseQueryError), "query subscribe promo candidates failed: %v", err)
-	}
-	return candidates, nil
+		Order("pr.id ASC")
 }
 
 func (c subscribePromoCandidate) isActive(now time.Time) bool {
@@ -179,11 +184,7 @@ func (e *promoEligibilityEvaluator) lastSubscribeExpireAt() (time.Time, error) {
 		return *e.lastExpire, nil
 	}
 	var item user.Subscribe
-	err := e.db.WithContext(e.ctx).
-		Model(&user.Subscribe{}).
-		Where("user_id = ?", e.userInfo.Id).
-		Where("expire_time != ?", time.UnixMilli(0)).
-		Order("expire_time DESC").
+	err := e.lastSubscribeExpireQuery().
 		Limit(1).
 		Take(&item).Error
 	if err != nil {
@@ -198,6 +199,18 @@ func (e *promoEligibilityEvaluator) lastSubscribeExpireAt() (time.Time, error) {
 	return item.ExpireTime, nil
 }
 
+func (e *promoEligibilityEvaluator) lastSubscribeExpireQuery() *gorm.DB {
+	return e.db.WithContext(e.ctx).
+		Model(&user.Subscribe{}).
+		Where("user_id = ?", e.userInfo.Id).
+		Order(clause.OrderBy{
+			Expression: clause.Expr{
+				SQL:  "CASE WHEN expire_time = ? THEN 0 ELSE 1 END, expire_time DESC",
+				Vars: []interface{}{time.UnixMilli(0)},
+			},
+		})
+}
+
 func (c subscribePromoCandidate) expiresAt() time.Time {
 	if c.EndTime == nil {
 		return time.Time{}