fix(api): a paused viewer keeps its HLS session, a refused subtitle publish backs off, and a NUL is a 422 everywhere

Deep-review round 4, backend half.

- The periodic HLS reaper (round 3) applied the "finished and unfetched for 30 s" rule that only a
  NEW session request may apply. A remux finishes long before the film, and a paused player with a
  full buffer fetches nothing, so a one-minute pause killed the session and playback died ~30 s after
  resuming. The periodic pass is `reap_abandoned` now (the long idle limit, finished or not).
- A subtitle `publish` that could not keep its output (full / read-only cache volume) returned
  quietly, so every poll queued the window and the whole-container pass again. It reports failure
  and the pass is backed off like a failed read; a cached VTT that cannot be READ is answered
  `unavailable` (`cache_unreadable`) instead of scheduling reads that can never land.
- The subtitle routes bound `rating_key` / `sub_ord` (they become a cache file name: a 250-digit
  one was ENAMETOOLONG, which pathlib's exists() re-raises -> a 500), and the cache helpers no longer
  throw on such a name.
- One app-wide handler turns psycopg's "text fields cannot contain NUL" DataError into a 422: round 3
  guarded only the by-rating_key lookup, and `contains=`, `contains_group=`, reorder/bulk key lists
  and the collection search still 500'd.
- The hand-written `db.get(...) is None -> 404 "Unknown ..."` siblings use `get_or_404` with a
  `what` (channel / video / user), so the Hungarian UI no longer shows English for a gone row.

backend/tests/test_hls_reaper.py

@@ -3,12 +3,20 @@
 plex/stream.py promises "a periodic reaper kills idle ones", but `reap_idle` ran only inside
 POST /session: after the last playback, an abandoned session (its ffmpeg still working through the
 film, its directory a whole segmented copy of it) lived until somebody pressed play again.
+
+And it must not take a PAUSED viewer's session: a remux finishes long before the film does, and a
+paused player with a full buffer fetches nothing — so "finished and unfetched for 30 s" (the rule a
+new session request applies) is not "abandoned".
 """
 
 import asyncio
+import time
+
+import pytest
 
 import app.main as app_main
 from app.main import app
+from app.plex import stream
 
 
 class _Configured:
@@ -31,7 +39,7 @@ def test_idle_sessions_are_reaped_while_nobody_starts_a_new_one(monkeypatch):
     monkeypatch.setattr(app_main, "shutdown_scheduler", lambda: None)
     monkeypatch.setattr(app_main.plex_subtitles, "shutdown", lambda: None)
     monkeypatch.setattr(app_main, "_HLS_REAP_EVERY_S", 0.01)
-    monkeypatch.setattr(app_main.plex_stream, "reap_idle", lambda: reaped.append(1) or 0)
+    monkeypatch.setattr(app_main.plex_stream, "reap_abandoned", lambda: reaped.append(1) or 0)
 
     async def _cycle():
         async with app_main.lifespan(app):
@@ -39,3 +47,60 @@ def test_idle_sessions_are_reaped_while_nobody_starts_a_new_one(monkeypatch):
 
     asyncio.run(_cycle())
     assert len(reaped) >= 2, "no periodic reap ran while the app was up"
+
+
+class _Proc:
+    def __init__(self, finished: bool):
+        self.finished = finished
+        self.killed = False
+
+    def poll(self):
+        return 0 if self.finished else None
+
+    def terminate(self):
+        self.killed = True
+
+    def wait(self, timeout=None):
+        return 0
+
+
+class _Session:
+    def __init__(self, tmp_path, name: str, finished: bool, idle_s: float):
+        self.dir = tmp_path / name
+        self.dir.mkdir()
+        self.proc = _Proc(finished)
+        self.last_access = time.time() - idle_s
+
+
+@pytest.fixture
+def sessions(monkeypatch):
+    live: dict = {}
+    monkeypatch.setattr(stream, "_sessions", live)
+    return live
+
+
+def test_the_periodic_pass_keeps_a_paused_viewers_finished_session(tmp_path, sessions):
+    # A remux that finished a minute ago, its viewer paused with a full buffer since.
+    s = _Session(tmp_path, "paused", finished=True, idle_s=90)
+    sessions["12345"] = s
+    assert stream.reap_abandoned() == 0
+    assert "12345" in sessions and s.dir.exists() and not s.proc.killed
+
+
+def test_the_periodic_pass_still_reaps_what_nobody_has_touched_for_the_idle_limit(
+    tmp_path, sessions
+):
+    sessions["1"] = _Session(
+        tmp_path, "gone_done", finished=True, idle_s=stream._SESSION_IDLE_S + 5
+    )
+    sessions["2"] = _Session(
+        tmp_path, "gone_live", finished=False, idle_s=stream._SESSION_IDLE_S + 5
+    )
+    assert stream.reap_abandoned() == 2
+    assert sessions == {}
+
+
+def test_a_new_session_request_still_reaps_finished_sessions_quickly(tmp_path, sessions):
+    sessions["12345"] = _Session(tmp_path, "done", finished=True, idle_s=45)
+    assert stream.reap_idle() == 1
+    assert sessions == {}