perf(downloads): de-prioritize worker transcodes + idle-back-off polls to stop LAN slowdown

The download/edit child (yt-dlp + ffmpeg) shares one LAN box with the API,
Postgres, and Plex playback. A CPU re-encode pegged every core for ~30s and
starved web requests + playback until it finished. Full-scope S4 mitigations:

- Child self-de-prioritizes at startup (inherited by every ffmpeg/yt-dlp
  descendant): CPU nice increment + best-effort idle ionice class + an
  OMP_NUM_THREADS budget, all config-gated and independently disable-able.
- Cap the browser-safe re-encode's encoder threads (`-threads`) so a transcode
  reserves a core for foreground work instead of claiming all of them.
- Throttle the retention GC's whole-tree orphan sweep (yield IO after each
  batch of files) so its rglob can't monopolize disk IO in the API process.
- Idle-back-off the Download Center (2s→15s when nothing queued/running) and
  UsageBar (2s→15s when running_jobs is 0) polls; both snap back to 2s on the
  next fetch after an enqueue (invalidateDownloads refetches immediately).

Tests: encode_thread_cap (pin/reserve-core/edge), -threads flag presence,
_deprioritize (nice/ionice/OMP branches), GC sweep throttle batching.

frontend/src/components/DownloadCenter.tsx

@@ -51,7 +51,13 @@ export default function DownloadCenter() {
   const [editJob, setEditJob] = useState<DownloadJob | null>(null);
   const [errorJob, setErrorJob] = useState<DownloadJob | null>(null);
 
-  const jobsQ = useLiveQuery(qk.downloads(), api.downloads, { intervalMs: 2000 });
+  // Poll fast (2s) while work is actually in flight; back off to 15s once the queue is settled
+  // (only done/error/paused rows left, which don't advance on their own) so an idle Download Center
+  // isn't hitting the API — and adding to the shared box's load — every 2s for nothing.
+  const jobsQ = useLiveQuery(qk.downloads(), api.downloads, {
+    intervalMs: (data) =>
+      data?.some((j) => j.status === "queued" || j.status === "running") ? 2000 : 15000,
+  });
   const sharedQ = useQuery({
     queryKey: qk.downloadsShared(),
     queryFn: api.downloadsShared,