fix(repo): preserve transfer recipient collaboration (#39042)

Remove temporary recipient access after a transfer ends while preserving
existing collaboration.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modelmigration/v28/v351.go

@@ -0,0 +1,23 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v28
+
+import (
+	"context"
+
+	"gitea.dev/modelmigration/base"
+
+	"xorm.io/xorm"
+)
+
+func AddRecipientAccessGrantedToRepoTransfer(_ context.Context, x base.EngineMigration) error {
+	type RepoTransfer struct {
+		RecipientAccessGranted bool `xorm:"NOT NULL DEFAULT false"`
+	}
+	_, err := x.SyncWithOptions(xorm.SyncOptions{
+		IgnoreConstrains:  true,
+		IgnoreDropIndices: true,
+	}, new(RepoTransfer))
+	return err
+}