fix: 修复兑换码搜索中已使用兑换码搜索不到的问题

将 SearchRedemptions 的 Count 和 Find 拆成独立查询链，
避免 GORM 事务中共用 query 对象导致 Count() 污染查询状态。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

model/redemption.go

@@ -74,25 +74,26 @@ func SearchRedemptions(keyword string, startIdx int, num int) (redemptions []*Re
 		}
 	}()
 
-	// Build query based on keyword type
-	query := tx.Model(&Redemption{})
-
-	// Only try to convert to ID if the string represents a valid integer
+	// Build WHERE condition and args based on keyword type
+	var whereClause string
+	var whereArgs []interface{}
 	if id, err := strconv.Atoi(keyword); err == nil {
-		query = query.Where("id = ? OR name LIKE ? OR "+commonKeyCol+" LIKE ?", id, keyword+"%", keyword+"%")
+		whereClause = "id = ? OR name LIKE ? OR " + commonKeyCol + " LIKE ?"
+		whereArgs = []interface{}{id, keyword + "%", keyword + "%"}
 	} else {
-		query = query.Where("name LIKE ? OR "+commonKeyCol+" LIKE ?", keyword+"%", keyword+"%")
+		whereClause = "name LIKE ? OR " + commonKeyCol + " LIKE ?"
+		whereArgs = []interface{}{keyword + "%", keyword + "%"}
 	}
 
 	// Get total count
-	err = query.Count(&total).Error
+	err = tx.Model(&Redemption{}).Where(whereClause, whereArgs...).Count(&total).Error
 	if err != nil {
 		tx.Rollback()
 		return nil, 0, err
 	}
 
 	// Get paginated data
-	err = query.Order("id desc").Limit(num).Offset(startIdx).Find(&redemptions).Error
+	err = tx.Model(&Redemption{}).Where(whereClause, whereArgs...).Order("id desc").Limit(num).Offset(startIdx).Find(&redemptions).Error
 	if err != nil {
 		tx.Rollback()
 		return nil, 0, err