feat(backup)!: opt-in hard restart and mandatory backups-dir

Rename --docker-compose-hard-restart-required to --hard-compose-restart
and change its default from ["mailu"] to [] (nargs="*"): the compose
down/up is now opt-in, so compose hosts pass "mailu" while swarm hosts,
where the dir is a stack whose overlay network collides with compose up,
pass nothing. Make --backups-dir mandatory (no /var/lib/backup/ default)
so a run can never silently target the wrong backup root.

BREAKING CHANGE: the old flag name is removed, the implicit mailu default
is gone, and --backups-dir must be passed explicitly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/baudolo/backup/app.py

@@ -247,8 +247,6 @@ def main() -> int:
     print("Finished volume backups.", flush=True)
 
     print("Handling Docker Compose services...", flush=True)
-    handle_docker_compose_services(
-        args.compose_dir, args.docker_compose_hard_restart_required
-    )
+    handle_docker_compose_services(args.compose_dir, args.hard_compose_restart)
 
     return 0