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/lib/types.ts

@@ -329,3 +329,59 @@ export interface TempUpload {
   created_at: string;
   updated_at: string;
 }
+
+export interface StatJob {
+  kind: "download" | "match";
+  id: string;
+  label: string;
+  status: string;
+  progress: number;
+  detail: string;
+  summary?: string;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface SystemStats {
+  system: {
+    cpu: {
+      percent: number;
+      per_cpu: number[];
+      cores: number;
+      physical_cores: number;
+      load: number[];
+    };
+    memory: {
+      total: number;
+      used: number;
+      available: number;
+      percent: number;
+      swap_total: number;
+      swap_used: number;
+      swap_percent: number;
+    };
+    uptime_seconds: number;
+  };
+  gpus: {
+    name: string;
+    utilization: number | null;
+    memory_used: number | null;
+    memory_total: number | null;
+    temperature: number | null;
+  }[];
+  disk: {
+    watched_folder: {
+      path: string;
+      total: number;
+      used: number;
+      free: number;
+      library_size: number;
+      percent_used: number;
+    };
+    media: { path: string; size: number };
+    temp: { path: string; size: number; files: number };
+    library_items: number;
+  };
+  jobs: { active: StatJob[]; recent: StatJob[]; active_count: number };
+  log: { path: string; lines: string[]; missing: boolean };
+}