Refactor Admin UI to use unified IAM storage and add MultipleFileStore (#8101)

* Refactor Admin UI to use unified IAM storage and add MultipleFileStore

* Address PR feedback: fix renames, error handling, and sync logic in FilerMultipleStore

* Address refined PR feedback: safe rename order, rollback logic, and structural sync refinement

* Optimize LoadConfiguration: use streaming callback for memory efficiency

* Refactor UpdateUser: log rollback failures during rename

* Implement PolicyManager for FilerMultipleStore

* include the filer_multiple backend configuration

* Implement cross-S3 synchronization and proper shutdown for all IAM backends

* Extract Admin UI refactoring to a separate PR

weed/iamapi/iamapi_server.go

@@ -129,6 +129,9 @@ func (iama *IamApiServer) Shutdown() {
 		glog.V(0).Infof("IAM API server shutting down, stopping master client connection")
 		iama.shutdownCancel()
 	}
+	if iama.iam != nil {
+		iama.iam.Shutdown()
+	}
 }
 
 func (iama *IamS3ApiConfigure) GetS3ApiConfiguration(s3cfg *iam_pb.S3ApiConfiguration) (err error) {