feat: 客户关联产品并支持按产品圈选通知群体

新增客户-产品多对多关联，列表可按产品筛选并复制邮箱；发证/下单自动回填历史关联。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/service/license/offline_service.go

@@ -4,6 +4,7 @@ import (
 	"backend/config"
 	"backend/internal/dto"
 	"backend/internal/model"
+	customersvc "backend/internal/service/customer"
 	"backend/utils"
 	"errors"
 	"fmt"
@@ -139,6 +140,7 @@ func (s *OfflineIssuer) Issue(req dto.OfflineLicenseCreate) (*dto.OfflineLicense
 	if err := s.db.Create(&lic).Error; err != nil {
 		return nil, errors.New("保存离线 License 失败")
 	}
+	_ = customersvc.AttachProduct(s.db, customer.ID, product.ID)
 
 	activation := model.Activation{
 		LicenseID:   lic.ID,