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/license_service.go

@@ -3,6 +3,7 @@ package license
 import (
 	"backend/internal/dto"
 	"backend/internal/model"
+	customersvc "backend/internal/service/customer"
 	"backend/utils"
 	"encoding/json"
 	"errors"
@@ -137,6 +138,7 @@ func (s *LicenseService) Create(req dto.LicenseCreate) ([]dto.LicenseResponse, e
 	if err := s.db.Create(&licenses).Error; err != nil {
 		return nil, errors.New("创建 License 失败")
 	}
+	_ = customersvc.AttachProduct(s.db, req.CustomerID, req.ProductID)
 
 	results := make([]dto.LicenseResponse, 0, len(licenses))
 	for _, l := range licenses {