GC never ran in the server's life: 13 ghost libraries, unlimited
history on 16/17 repos, 867G blocks vs 710G live data, and an
fs/commits object swarm that makes per-file backup walks take days.
Procedure: pre-GC zfs snapshot, keep_days=90 default, offline
seaf-gc (+ --rm-deleted, --rm-fs), measure, quarterly GC schedule,
then re-decide the S3 backup mechanism and the 13.x upgrade.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
