Native SFTP backend for Restic backups

[konradx64]

Dockhand currently supports Local, S3, B2, Azure, GCS and REST destinations, but Restic itself also supports SFTP natively.

Native SFTP support would be especially useful for common backup targets such as Hetzner Storage Box, NAS systems and generic SSH backup servers, without requiring SSHFS, rclone bridges or a separate REST server.

Ideally, the destination UI could support:

Host / port
Username
SSH private key
Optional key passphrase
Host key / known_hosts verification
Remote repository path

Example Restic repository:
sftp://user@host:23/path/to/repo