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

- the sweep guard anchors the rating_key to digits: the round-2 widening had
  degenerated to "anything ending in _<digits>" (backup_2024, release_10)
- RSS counts apply-side failures too, so a read-only database no longer reports
  a wholly failed poll as "ok — new=0, failed=0"
- the player reads/writes its volume directly (readAccount/writeAccount): the
  persisted-object hook rendered nothing and cost a second write per settle
- new tests: the sweep guard's accept/reject table, and run_ffmpeg's threading
  (drain, cancel while silent, stall watchdog, stderr tail on a nonzero exit)
- the breaker sentinel is checked one way (isinstance) at all four sites
- run_rss_poll is typed dict[str, int]

All three new test groups were mutation-checked: reverting each fix turns them
red (or, for the cancel/stall pair, hangs — which is the bug itself).

frontend/src/components/PlayerModal.tsx

@@ -35,7 +35,7 @@ import {
 } from "../lib/format";
 import { renderDescription } from "../lib/descriptionLinks";
 import { useBackToClose } from "../lib/history";
-import { LS, useAccountPersistedObject, writeAccount } from "../lib/storage";
+import { LS, readAccount, writeAccount } from "../lib/storage";
 import { useScrollFade } from "../lib/useScrollFade";
 
 // Experiment (branch experiment/inline-player): play the video in-app via the
@@ -183,15 +183,12 @@ export default function PlayerModal({
   // always starts at full volume. Per-account, like the Plex player's prefs. Level 0 IS the muted
   // state (that's how YouTube's own control behaves), so there's no separate `muted` flag to
   // contradict it.
-  const [playerPrefs, patchPlayerPrefs] = useAccountPersistedObject(LS.ytPlayerPrefs, {
-    volume: 100,
-  });
-  // The live volume: updated on every notch, while the localStorage write behind it is debounced
-  // (a wheel spin fires ~16 events a second, and each patch is a synchronous setItem on the frame
-  // the player is decoding on). Everything that READS the level reads this ref, so it never sees
-  // the stale persisted value mid-spin. The window/wheel listeners are bound once, so a ref — not
-  // state — is what they can safely close over.
-  const volumeRef = useRef(playerPrefs.volume);
+  //
+  // Ref + explicit read/write rather than `useAccountPersistedObject`: nothing RENDERS the level
+  // (the on-player flash has its own `volumeUi` state), the once-bound wheel/key listeners need a
+  // ref anyway, and the hook's write lives inside a `setState` updater — which is both a second
+  // write on every settle and not something React must run while the component is unmounting.
+  const volumeRef = useRef(readAccount(LS.ytPlayerPrefs, { volume: 100 }).volume);
   const volSaveTimerRef = useRef<number | undefined>(undefined);
   // When the last volume gesture happened — see nudgeVolume (a spin must not re-read the player).
   const lastNudgeAtRef = useRef(0);
@@ -215,18 +212,14 @@ export default function PlayerModal({
     window.clearTimeout(volTimerRef.current);
     volTimerRef.current = window.setTimeout(() => setVolumeUi(null), 1200);
   };
-  /** Push a level (0–100) to the player and remember it; 0 also mutes (a player left at zero
-   * volume but unmuted shows YouTube's speaker icon as ON, which reads as broken). */
+  /** Write the current level out now, cancelling any pending debounce. */
   const persistVolume = () => {
     window.clearTimeout(volSaveTimerRef.current);
     volSaveTimerRef.current = undefined;
-    // Write STRAIGHT to storage, not through patchPlayerPrefs: that helper persists inside a
-    // `setState` updater, and React is under no obligation to run an updater for a component that
-    // is unmounting — which is exactly when the flush below fires (close the player right after a
-    // wheel spin). The state copy is only needed while mounted, so patch that separately.
     writeAccount(LS.ytPlayerPrefs, { volume: volumeRef.current });
-    patchPlayerPrefs({ volume: volumeRef.current });
   };
+  /** Push a level (0–100) to the player and remember it; 0 also mutes (a player left at zero
+   * volume but unmuted shows YouTube's speaker icon as ON, which reads as broken). */
   const applyVolume = (level: number, flash = true) => {
     const next = Math.max(0, Math.min(100, Math.round(level)));
     const p = playerRef.current;