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/sftpd/user/filestore.go

@@ -264,3 +264,8 @@ func (s *FileStore) CreateUser(username, password string) (*User, error) {
 
 	return user, nil
 }
+
+// Reload reloads users from the file, useful for HUP signal handling
+func (s *FileStore) Reload() error {
+	return s.loadUsers()
+}