fix(plex): a subtitle failure about one position no longer ends the track, and windows are read ahead and never wait behind a full pass

Deep-review round 3 of bug hunt 2026-09.

- `failed` is now final only for a track that can never be text (bitmap, no item/track/file). A Plex
  hiccup or a stretch ffmpeg could not read answers `unavailable`: said, polled again every 5 s, and
  the <track> stays mounted when cues exist elsewhere (gen > 0). Re-picking the selected track asks
  the server again at once.
- Within LOOKAHEAD_S of a covered stretch's end the next window is read ahead, instead of only once
  the playhead has fallen off it (subtitles dropped out at every edge while the full pass was backed
  off or slower than a window lasts).
- Windows and full passes run on separate pools with separate in-flight caps, so a window never
  queues behind (or is refused by) minute-long full passes — the pool comment promised that and
  nothing enforced it.
- A pass that times out is TERMinated and keeps what it reached (published as partial) instead of
  being killed and discarded.
- Lapsed failure memos are swept on every write (they were only pruned when the same key was read
  again, so polling fresh keys grew the process without bound).
- `publish` no longer merges into a sidecar with no VTT behind it, or claims a region whose VTT write
  failed, or overwrites an unreadable track with one window.
- A NUL in a rating_key names no row (it was a DataError, a 500, on every Plex route) and no longer
  500s the status poll.

Tests: 16 new, each mutation-checked (18 mutations, all red on the test that names them).

backend/tests/test_plex_subtitle_auth.py

@@ -35,7 +35,8 @@ def test_a_suspended_account_cannot_drive_the_subtitle_extractor(
         monkeypatch.setattr(mod, "resolved_user_id", lambda request: (user.id, True))
     monkeypatch.setattr(subtitles, "CACHE_DIR", tmp_path)
     pool = _NoPool()
-    monkeypatch.setattr(subtitles, "_POOL", pool)
+    monkeypatch.setattr(subtitles, "_WIN_POOL", pool)
+    monkeypatch.setattr(subtitles, "_FULL_POOL", pool)
     subtitles.reset_state()
 
     client = TestClient(app)