On the 38 GB ops disk the s3fs cache reserve (ensure_diskfree) never
fit, so all three mounts failed at every boot since 2026-09-22. Backups
now go to Hetzner S3 with rclone instead.
Add s3fs_enabled (default true); when false the role unmounts the
buckets and drops their fstab entries, keeping the credentials file
that the rclone wrapper reads.
Assisted-by: Claude:opus-5
