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.

backend/app/downloads/gc.py

@@ -86,11 +86,25 @@ def _sweep_orphan_files(db: Session, download_root: str) -> int:
 
     cutoff = time.time() - _ORPHAN_MIN_AGE_S
     removed = 0
+    # Throttle the whole-tree walk: this runs in the API process, and an unpaced rglob+stat over a
+    # large download root monopolizes disk IO long enough to make web requests + playback stutter.
+    # Yield the IO channel after every `batch` files touched so foreground work interleaves.
+    pause_s = max(0, settings.download_gc_sweep_pause_ms) / 1000.0
+    batch = max(1, settings.download_gc_sweep_batch)
+    touched = 0
+
+    def _throttle() -> None:
+        nonlocal touched
+        touched += 1
+        if pause_s and touched % batch == 0:
+            time.sleep(pause_s)
+
     for entry in root.iterdir():
         if entry.name.startswith("."):  # .edits / .plex-* / .staging / .storyboards + any dotfile
             continue
         files = [entry] if entry.is_file() else [p for p in entry.rglob("*") if p.is_file()]
         for f in files:
+            _throttle()
             try:
                 if f.resolve() in keep or f.stat().st_mtime > cutoff:
                     continue