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.

backend/requirements.txt

@@ -7,3 +7,4 @@ requests>=2.32
 imagehash>=4.3
 PyMySQL>=1.1
 redis>=5.0
+psutil>=6.0