repository maintenance: implement admin gc, scheduled repack, admin stats #27

gitbayd admin gc is still a not-implemented stub, and nothing ever repacks or prunes repositories — imported repos carry whatever GitHub sent, and busy repos accumulate loose objects forever.

• implement admin gc [--repo owner/name]: git gc/repack with sane flags across all or one repo
• optional scheduled maintenance (systemd timer like the backup, or a [maintenance] config knob)
• report disk usage per repo while at it (admin stats is also a stub)

Live: admin stats (66 repos, 198.3 MiB before gc, 872 KiB database) and the first full admin gc freed 6.4 MiB. gitbay-gc.timer enabled on bay1, next run Sun 2026-08-30 07:13 UTC; cloud-init provisions it for fresh installs. Post-gc clone verified in e2e.