SFTP: support reloading user store on HUP signal (#7651)

Fixes #7650

This change enables the SFTP server to reload the user store configuration
(sftp_userstore.json) when a HUP signal is sent to the process, without
requiring a service restart.

Changes:
- Add Reload() method to FileStore to re-read users from disk
- Add Reload() method to SFTPService to handle reload requests
- Register reload hook with grace.OnReload() in sftp command

This allows administrators to add users or change access policies
dynamically by editing the user store file and sending a HUP signal
(e.g., 'systemctl reload seaweedfs' or 'kill -HUP <pid>').

weed/command/sftp.go

@@ -16,6 +16,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/sftpd"
 	stats_collect "github.com/seaweedfs/seaweedfs/weed/stats"
 	"github.com/seaweedfs/seaweedfs/weed/util"
+	"github.com/seaweedfs/seaweedfs/weed/util/grace"
 )
 
 var (
@@ -148,6 +149,9 @@ func (sftpOpt *SftpOptions) startSftpServer() bool {
 		UserStoreFile:       *sftpOpt.userStoreFile,
 	})
 
+	// Register reload hook for HUP signal
+	grace.OnReload(service.Reload)
+
 	// Set up Unix socket if on non-Windows platforms
 	if runtime.GOOS != "windows" {
 		localSocket := *sftpOpt.localSocket