refactor: Apply further sdm improvements — device-path unmount and remove redundant syncs

[Copilot]

After the recent storage-device-managers update, a few more manual operations can be delegated to sdm.

Changes

• close command — device path for unmount: sdm.unmount_device() was called with the mount directory, which caused sync_device inside it to fall back to a generic sync -f (since blkid fails on directories). Now passes cfg.map_name() (the /dev/mapper/<UUID> path), enabling proper filesystem-aware sync — including BtrFS-specific btrfs filesystem sync. The now-unnecessary mount_dir extraction is removed.

• Remove BtrFSRsyncBackend.sync_filesystem_changes: The explicit btrfs filesystem sync at the end of do_backup is redundant — sdm.mounted_device already calls unmount_device(decrypted) on exit, which runs sync_device with the actual device path, covering both the BtrFS-specific sync and sync -f.

• Remove ResticBackend.sync_filesystem_changes: Same reasoning — the sync -f backup_repository at the end of copy_files is covered by the context manager's teardown sequence.