feat(api): add on-demand world backup endpoint and Job executor (§B4 Sync)

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.

internal/api/backuper_wire_test.go

@@ -0,0 +1,12 @@
+package api
+
+import (
+	"felis.lolicon.best/internal/backupjob"
+)
+
+// Compile-time proof that the production backup executor satisfies the API's
+// Backuper interface, mirroring restore_wire_test. The dependency points one way
+// only: api defines the narrow Backuper port and never imports backupjob in
+// production (handlers_backups.go depends on the interface); this test is the single
+// place the concrete type and the port are pinned together.
+var _ Backuper = (*backupjob.Backuper)(nil)