fix(plex): subtitles — a retried full pass resumes, a stalled track is polled slowly

A full pass that ran out of time kept what it reached, but its retry an hour
later started again at 0, so a file too big to read inside the extraction
timeout never completed. The pass now starts where every embedded track's
coverage from the head ends (less the window pre-roll, so a cue across the
join is read whole). The seek needs -avoid_negative_ts on this read as on a
window, measured on a real file: without it the cue running at the start
shifts the whole output.

A partial track whose full pass had failed never reached a settled state, so
the player polled it every 2 s for the rest of the film. The status now says
whether the track is still growing, and a stalled one is polled every 15 s —
still inside the 60 s lookahead that reads the next window before an edge.

frontend/src/lib/plexPlayerHooks.ts

@@ -484,6 +484,11 @@ const SUB_POLL_ERROR_MS = 10_000;
 // server keeps for a minute), so no need for the 1 s cadence — but the poll carries the position, and
 // moving into a stretch that IS cached should stop saying "couldn't be loaded" within a few seconds.
 const SUB_POLL_UNAVAILABLE_MS = 5_000;
+// …and on a `partial` track that is no longer `growing` (its full pass failed and is backed off for
+// an hour): it will not fill in on its own, so the 2 s cadence ran for the rest of the film. The poll
+// still carries the position — it is what reads the next window ahead of an edge, which the server
+// starts within LOOKAHEAD_S (60 s) of it — so it slows down rather than stops.
+const SUB_POLL_STALLED_MS = 15_000;
 const SUB_TRACK_RETRIES = 2;
 const SUB_TRACK_RETRY_MS = 2_000;
 
@@ -684,7 +689,8 @@ export function usePlexTracks(opts: {
       const st = q.state.data?.state;
       if (st === "ready" || st === "failed") return false;
       if (st === "unavailable") return SUB_POLL_UNAVAILABLE_MS;
-      return st === "partial" ? 2000 : 1000;
+      if (st === "partial") return q.state.data?.growing === false ? SUB_POLL_STALLED_MS : 2000;
+      return 1000;
     },
     staleTime: 0,
     retry: false,