fix: 离线证到期重签空邮箱撞唯一索引

复用 customer_id/同名客户，禁止重复插入空 email；重签前端带回原客户。

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

backend/internal/service/license/license_service.go

@@ -304,8 +304,12 @@ func (s *LicenseService) ExportCSV(q ListQuery) (string, error) {
 // toLicenseResponse 模型转响应 DTO // Logan.Li
 func toLicenseResponse(l model.License) dto.LicenseResponse {
 	customerName := ""
+	customerEmail := ""
 	if l.Customer.ID > 0 {
 		customerName = l.Customer.Name
+		if !strings.HasSuffix(l.Customer.Email, "@local.invalid") {
+			customerEmail = l.Customer.Email
+		}
 	}
 	productName := ""
 	if l.Product.ID > 0 {
@@ -332,6 +336,7 @@ func toLicenseResponse(l model.License) dto.LicenseResponse {
 		DeviceID:           deviceID,
 		CustomerID:         l.CustomerID,
 		CustomerName:       customerName,
+		CustomerEmail:      customerEmail,
 		ProductID:          l.ProductID,
 		ProductName:        productName,
 		Status:             l.Status,