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/tests/test_sync_guards.py

@@ -51,6 +51,11 @@ class TestBreaker:
         assert not b.open()
 
 
+def _quota_dead():
+    """A write op that fails the way a quota-exhausted / scope-revoked account fails."""
+    raise YouTubeError("quota exceeded")
+
+
 class TestBreakerRun:
     def test_returns_the_ops_result_and_leaves_failures_alone(self):
         b, failed = _Breaker(), []
@@ -59,22 +64,25 @@ class TestBreakerRun:
 
     def test_a_youtube_error_records_the_failure_and_counts_towards_the_trip(self):
         b, failed = _Breaker(limit=2), []
-        assert b.run(lambda: (_ for _ in ()).throw(YouTubeError("quota")), lambda: failed.append("v1")) is FAILED
+        assert b.run(_quota_dead, lambda: failed.append("v1")) is FAILED
         assert failed == ["v1"] and b.open()
-        b.run(lambda: (_ for _ in ()).throw(YouTubeError("quota")), lambda: failed.append("v2"))
+        b.run(_quota_dead, lambda: failed.append("v2"))
         assert not b.open()
 
     def test_a_tripped_breaker_never_calls_the_op(self):
         b, failed, calls = _Breaker(limit=1), [], []
-        b.run(lambda: (_ for _ in ()).throw(YouTubeError("quota")), lambda: failed.append("v1"))
+        b.run(_quota_dead, lambda: failed.append("v1"))
         assert b.run(lambda: calls.append("called"), lambda: failed.append("v2")) is FAILED
         assert calls == []  # the whole point: no more quota spent
         assert failed == ["v1", "v2"]  # …but the item is still reported
 
     def test_other_exceptions_are_not_swallowed(self):
         # Only YouTubeError is a "write failed" signal; a bug must still surface.
+        def boom():
+            raise ValueError("bug")
+
         with pytest.raises(ValueError):
-            _Breaker().run(lambda: (_ for _ in ()).throw(ValueError("bug")), lambda: None)
+            _Breaker().run(boom, lambda: None)
 
 
 class TestRssErrorVsEmpty: