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/common/promoEligibility.go

@@ -11,6 +11,7 @@ import (
 	"github.com/perfect-panel/server/pkg/xerr"
 	"github.com/pkg/errors"
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 )
 
 type PromoResult struct {
@@ -148,7 +149,12 @@ func evaluateInactiveUserPromo(
 	err := db.WithContext(ctx).
 		Model(&user.Subscribe{}).
 		Where("user_id = ?", userID).
-		Order("expire_time DESC").
+		Order(clause.OrderBy{
+			Expression: clause.Expr{
+				SQL:  "CASE WHEN expire_time = ? THEN 0 ELSE 1 END, expire_time DESC",
+				Vars: []interface{}{time.UnixMilli(0)},
+			},
+		}).
 		Limit(1).
 		Take(&lastSub).Error
 	if err != nil {
@@ -158,8 +164,16 @@ func evaluateInactiveUserPromo(
 		return false, time.Time{}, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseQueryError), "query promo inactive user subscription failed")
 	}
 
+	return evaluateInactiveUserExpire(lastSub.ExpireTime, params, now), ruleExpiresAt, nil
+}
+
+func evaluateInactiveUserExpire(lastExpire time.Time, params promoRuleParams, now time.Time) bool {
+	if lastExpire.Equal(time.UnixMilli(0)) {
+		return false
+	}
+
 	threshold := now.AddDate(0, -params.InactiveMonths, 0)
-	return lastSub.ExpireTime.Before(threshold) || lastSub.ExpireTime.Equal(threshold), ruleExpiresAt, nil
+	return lastExpire.Before(threshold) || lastExpire.Equal(threshold)
 }
 
 func promoRuleExpiresAt(rule *promo.RuleWithPrice) time.Time {