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.

backend/tests/test_plex_subtitle_prepare.py

@@ -395,6 +395,17 @@ def test_polls_that_follow_the_playhead_join_one_window(monkeypatch):
     assert len(pool.jobs) == 2, "one window + the one full pass"
 
 
+def test_a_window_for_a_different_stretch_does_not_wait_behind_the_first(monkeypatch):
+    """Measured on localdev: after a resume the first poll can still report 0 (the position is only
+    known once playback ticks), so the first window is cut around the wrong spot. The poll that
+    follows, at the real position, must get its own window at once — not queue behind that one."""
+    pool = _ManualPool()
+    monkeypatch.setattr(subtitles, "_POOL", pool)
+    subtitles.ensure("98614", 1, at=0.0)
+    subtitles.ensure("98614", 1, at=1198.0)
+    assert sorted(a[3] for _, a in pool.jobs if a[3] is not None) == [0.0, 1168.0]
+
+
 def test_a_queued_window_that_was_covered_meanwhile_reads_nothing(monkeypatch):
     """Every embedded track rides each pass over its file, so by the time a queued window for one
     language runs, another language's window may already have published it."""