fix(r5): address the second /code-review high round — 7 findings

- the volume keys no longer defer to a scrollable card while in fullscreen
  (mirrored into a ref: the keydown handler is bound once and state would be stale)
- the debounced volume flush writes straight to storage; persisting inside a
  setState updater is not guaranteed to run for an unmounting component
- the HLS sweep also matches the original {key}_{start} directory naming, which
  is what the oldest leftovers on disk actually use
- run_rss_poll returns {new, failed} so an egress outage stops rendering as
  "ok — 0" in the scheduler card and the audit log; /api/sync/rss reports it too
- drop the unused poll_rss_channel (it silently gained a new exception)
- _Breaker.run is generic, so a call site keeps its item-id type
- test helper instead of the generator-throw idiom

backend/app/sync/videos.py

@@ -14,7 +14,6 @@ from app import progress, sysconfig
 from app.models import LIVE_OR_UPCOMING, Channel, Video
 from app.titles import normalize_title
 from app.youtube.client import YouTubeClient, best_thumbnail
-from app.youtube.rss import fetch_channel_feed
 from app.youtube.shorts import make_client, probe_is_short
 
 _DURATION_RE = re.compile(
@@ -84,10 +83,6 @@ def apply_rss_feed(db: Session, channel: Channel, entries: list[dict]) -> int:
     return inserted
 
 
-def poll_rss_channel(db: Session, channel: Channel) -> int:
-    return apply_rss_feed(db, channel, fetch_channel_feed(channel.id))
-
-
 # --- Backfill (uploads playlist; costs quota) ---
 def _stub_from_playlist_item(item: dict, channel_id: str) -> dict | None:
     content = item.get("contentDetails", {})