fix(plex): the subtitle poll backs off on errors, a failed <track> is replaced, and a resume asks for the right window

Deep-review R2, plus one defect found while verifying it on localdev.

- The status poll re-armed every second after a FAILED request: `data` stays empty on an error,
  so it read as "still preparing" for as long as the player was open — and the request was not
  quiet, so a persistent 500 re-opened the global error dialog every second. It now backs off to
  10 s (a blip still heals on its own) and is quiet; the player already shows the failure inline.
- A <track> whose own VTT fetch failed never fetched again: with the server's answer unchanged its
  key was unchanged, so React kept the dead element — silently blank subtitles. The key (now built
  in the hook, where it is tested) carries a failed-load count; after two remounts the track is
  reported as unavailable instead.
- Found in UAT: until the first timeupdate `abs` is 0, so after a resume every poll said at=0 and
  the first window was cut around the head of the film (a resume at 20:00 polled at=0 for as long
  as it stayed paused). The poll reports the session's start until playback ticks. And the window's
  dedup identity is the track plus a window-length bucket of its start — keyed on the track alone
  (R1), the right window queued behind that wrong one.

The poll's start/stop rules finally have tests (every earlier one resolved a single fixed answer).
Verified on localdev against Westworld S1E6 resumed at 20:00: the polls carried at=1197, one
subtitle line on screen at 21:02, the poll stopped at `ready`; the real file's window (56 cues) +
full pass (608) merged to 608 — the old merge would have served 664. 11 mutations, all red.

frontend/src/components/PlexPlayer.tsx

@@ -380,6 +380,7 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
     subShowable,
     subFail,
     subGen,
+    subTrackKey,
     onSubTrackError,
   } = usePlexTracks({
     id,
@@ -873,10 +874,10 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
             above flips modes; the browser fetches a track's VTT the first time it's shown. */}
         {selectedSub && subShowable && (
           <track
-            // One track = the selected sub only (no multi-track overlap). Key by the shifted-offset so
-            // a seek-restart or a (debounced) sub-offset change remounts → fresh VTT fetch, and by the
-            // extraction generation so a widened track replaces the first short window.
-            key={`${id}-${selectedSub.ord}-${subTrackOffset.toFixed(2)}-${subGen}`}
+            // One track = the selected sub only (no multi-track overlap). The key (see usePlexTracks)
+            // is what remounts it for a fresh VTT fetch: a seek-restart, an offset change, a widened
+            // track, or a failed load.
+            key={subTrackKey}
             kind="subtitles"
             src={api.plexSubtitleUrl(id, selectedSub.ord, subTrackOffset, subGen)}
             srcLang={selectedSub.language ?? undefined}