Add POST /api/v1/servers/{name}/backup: an owner or admin snapshots a
stopped server's world into the archive store on demand, recorded as a
first-class world_backups row (reason `manual`) — restorable by the
existing restore path and expired by the reaper's retention pass, so it
never leaks as an orphan archive. This is the break-glass "Sync" op,
resolved as immediate/on-demand backup.
felis-api cannot archive in-process (the world PVC is RWO, held by the
operator StatefulSet), so the work hands off to a one-shot Kubernetes Job
(new internal/backupjob) that mounts the world PVC read-only and the
backup PVC read-write, plus the felis config Secret so it self-records
its row atomically like the reaper. The Pod mirrors restore's weak-SA
isolation (SA token un-mounted, non-root, read-only rootfs, drop ALL);
the one reviewed departure is that config-Secret mount, frozen by
jobspec_test.go. Handler answers 202 backing_up; gated on the server
being Stopped (RWO world PVC), owner-or-admin, and FELIS_IMAGE +
FELIS_BACKUP_PVC being wired (else 503 backup_unavailable).
Each request mints a unique Job name (backup-<server>-<rand>) so a repeat
on-demand backup produces a fresh archive rather than colliding with a
just-finished Job still inside its TTL window and silently no-op'ing the
retry.
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