fix(plex): the subtitle extractor's seams — doubled cues, a truncated track cached as whole, a poll that respawned ffmpeg

Deep-review of the bug-hunt round (R1). The background extractor from ed4ab4e1 held, but its seams
did not:

- Window + full pass doubled every cue in the first hour. merge_vtt de-duplicated on the raw timing
  text, and the two passes spell a timestamp differently (shift_vtt writes HH:MM:SS.mmm, ffmpeg's
  webvtt muxer drops the hour below 1 h; shift_vtt(-0.0) never normalised the full pass). Timing is
  now parsed and re-emitted canonically, keyed with the cue text so two cues sharing a time survive.
- A full pass that exited non-zero was published as complete: "ready" forever, subtitles ending
  where ffmpeg died, and no eviction to ever retry. It now covers only up to its last cue, is never
  complete, and records the failure memo.
- The window's dedup identity held the live playhead, so every 1 s poll queued another read of the
  file. It is keyed on the track now, and a queued job re-checks coverage before reading anything.
- A sidecar track waited behind an unrelated embedded full pass (the full job is per item). Its
  window job now fetches it whole — one small request.
- The <track> fetch ran the scheduler at position 0: an unrequested window over the head of the file,
  and a refusal of a window that was on disk. It is a pure read of the cache now.
- publish() is serialised (a lost update became permanent once complete was set); the pool is shut
  down and running ffmpeg children killed at app shutdown (they held interpreter exit for minutes);
  ?at=inf is a 422, not a 500; and both subtitle routes use current_user instead of the cookie-only
  poster gate, which let a suspended or epoch-revoked session keep scheduling reads (pre-existing).

_extract_and_publish finally has tests of its own — every earlier test stubbed it out. 19 mutations,
each red on the assertion that names it.

backend/tests/test_plex_subtitle_auth.py

@@ -0,0 +1,44 @@
+"""The subtitle routes must refuse a session the rest of the app refuses.
+
+They used to sit behind `_require_session`, the cookie-only gate the poster proxies use — a signed
+cookie was enough, with none of `current_user`'s checks (is_active, is_suspended, the session epoch
+that "log out everywhere" bumps). Fine for a poster; not for a route that starts reads of the media
+library. Found by the 2026-09 bug-hunt deep-review; the gate itself pre-dates that round.
+"""
+
+from fastapi.testclient import TestClient
+
+import app.auth as auth
+import app.state as app_state
+from app.db import get_db
+from app.main import app
+from app.plex import subtitles
+from app.routes.plex import playback
+
+
+class _NoPool:
+    def __init__(self) -> None:
+        self.jobs: list = []
+
+    def submit(self, fn, *args) -> None:
+        self.jobs.append((fn, args))
+
+
+def test_a_suspended_account_cannot_drive_the_subtitle_extractor(
+    db, make_user, override_dep, monkeypatch, tmp_path
+):
+    user = make_user(is_suspended=True)
+    override_dep(get_db, lambda: db)
+    monkeypatch.setattr(app_state, "_configured_cache", True)
+    # A validly SIGNED cookie for that account — which is all the old gate looked at.
+    for mod in (auth, playback):
+        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)
+    subtitles.reset_state()
+
+    client = TestClient(app)
+    assert client.get("/api/plex/subtitle/98614/0/status?at=600").status_code == 401
+    assert client.get("/api/plex/subtitle/98614/0").status_code == 401
+    assert pool.jobs == [], "a refused session still scheduled an extraction"