host(koumakan): make backup actually own backup

Changed files
+1
systems
koumakan
services
storage
+1
systems/koumakan/services/storage/backup.nix
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${lib.getExe pkgs.btrfs-progs} subvolume create /home/backup
fi
+
chown backup /home/backup
chmod 0550 /home/backup
install -dm755 /home/backup/public
install -dm700 /home/backup/private