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/dto/license.go

@@ -32,6 +32,7 @@ type LicenseResponse struct {
 	DeviceID           string `json:"device_id,omitempty"` // 离线证绑定设备
 	CustomerID         uint   `json:"customer_id"`
 	CustomerName       string `json:"customer_name"`
+	CustomerEmail      string `json:"customer_email,omitempty"`
 	ProductID          uint   `json:"product_id"`
 	ProductName        string `json:"product_name"`
 	Status             string `json:"status"`