Add the staff stats dashboard

Backend: GET /api/stats/ (staff only) gathers psutil CPU/memory counters,
nvidia-smi GPU stats, the cached disk numbers and the running/finished
download + match jobs. Root logging now also writes a rotating file
(backend/logs/j621.log) so the dashboard can tail it, and psutil joins the
requirements. The storage payload computation is shared with the existing
storage endpoint.

Frontend: a /stats route + Stats nav entry for staff, polling every 2 s —
per-core CPU bars, memory and swap, GPUs (utilization, VRAM, temperature),
disk with the media/temp breakdown, active jobs with progress bars,
recently finished jobs with summaries, and the log tail with level colours
and an auto-scroll toggle. Section 4 of the roadmap is complete.

frontend/src/components/AppShell.tsx

@@ -71,7 +71,10 @@ export function AppShell() {
         ]
       : []),
     ...(user && (user.is_staff || user.is_superuser || user.role === "staff")
-      ? [{ to: "/users", label: "Users" }]
+      ? [
+          { to: "/stats", label: "Stats" },
+          { to: "/users", label: "Users" },
+        ]
       : []),
   ];