feat(api): admin system metrics endpoint with disk/mem/cpu alerts (0.40.0)

Adds GET /api/v1/admin/system returning disk (scoped to storage_dir
via statvfs), memory, CPU, and a server-side alerts array that fires
at >90% disk or memory.

Disk uses nix::sys::statvfs directly rather than sysinfo's Disks API
to avoid mountpoint-matching gymnastics for the storage_dir. A new
`Storage::local_root() -> Option<&Path>` trait method exposes the
root; the default returns None so a future S3Storage gets `disk:
null` in the response instead of fabricated numbers.

CPU is sampled inline (refresh → 250ms sleep → refresh → read) so the
endpoint adds 250ms of latency per call. No background-cache yet —
admin traffic is low-volume and the moving parts aren't worth it
until polling shows up.

Alerts are evaluated server-side so the frontend can render them
without re-implementing the thresholds.

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.39.0",
+  "version": "0.40.0",
   "private": true,
   "type": "module",
   "scripts": {