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/apps/core/views.py

@@ -1,10 +1,12 @@
 import time
 
 from django.conf import settings
-from rest_framework.permissions import AllowAny
+from rest_framework.exceptions import PermissionDenied
+from rest_framework.permissions import AllowAny, IsAuthenticated
 from rest_framework.response import Response
 from rest_framework.views import APIView
 
+from . import stats
 from .system_info import get_os_info
 
 
@@ -40,3 +42,27 @@ class StatusView(APIView):
         }
         payload["server_time_ms"] = round((time.perf_counter() - started) * 1000, 1)
         return Response(payload)
+
+
+class StatsView(APIView):
+    """Staff-only runtime statistics for the dashboard."""
+
+    permission_classes = [IsAuthenticated]
+
+    def get(self, request):
+        user = request.user
+        if not (
+            user.is_staff or user.is_superuser or user.role == user.ROLE_STAFF
+        ):
+            raise PermissionDenied("Staff only.")
+        from apps.library.tools import storage_info
+
+        return Response(
+            {
+                "system": stats.system_stats(),
+                "gpus": stats.gpu_stats(),
+                "disk": storage_info(),
+                "jobs": stats.jobs_stats(),
+                "log": stats.log_tail(),
+            }
+        )