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/app/main.py

@@ -24,6 +24,7 @@ log = logging.getLogger("siftlode.app")
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import FileResponse, HTMLResponse, JSONResponse
 from fastapi.staticfiles import StaticFiles
+from sqlalchemy.exc import DataError
 from starlette.middleware.sessions import SessionMiddleware
 
 from app import state
@@ -109,11 +110,13 @@ async def _reap_hls_sessions() -> None:
     """The periodic reaper plex/stream.py's docstring promises. `reap_idle` used to run only inside
     the NEXT session request, so after the last playback of the day an abandoned session — its
     ffmpeg still working through the film, its directory a whole segmented copy of it — lived until
-    somebody pressed play again or the process restarted."""
+    somebody pressed play again or the process restarted.
+
+    `reap_abandoned`, not `reap_idle`: a finished session is also a paused viewer's."""
     while True:
         await asyncio.sleep(_HLS_REAP_EVERY_S)
         try:
-            await asyncio.to_thread(plex_stream.reap_idle)
+            await asyncio.to_thread(plex_stream.reap_abandoned)
         except Exception:
             log.exception("HLS session reap failed")
 
@@ -147,6 +150,21 @@ async def _validation_error(request: Request, exc: RequestValidationError) -> JS
     )
 
 
+@app.exception_handler(DataError)
+async def _nul_in_text(request: Request, exc: DataError) -> JSONResponse:
+    """A NUL byte in a string the request sent is a 422, on every route at once. Postgres cannot hold
+    NUL in text, so the driver refuses to even send the query — and a `%00` in a query parameter or
+    an escaped NUL in a JSON body (a playlist's `contains=`, a reorder's key list, a collection
+    search) was a 500 with a traceback wherever it reached a comparison. Guarding each site did not
+    hold: round 3 guarded the by-rating_key lookup and missed the rest. Any other DataError is left
+    alone — re-raised, it stays the 500 it always was."""
+    if "NUL (0x00)" not in str(exc.orig):
+        raise exc
+    return JSONResponse(
+        status_code=422, content={"detail": {"code": "invalid_input", "reason": "nul"}}
+    )
+
+
 app.add_middleware(
     SessionMiddleware,
     secret_key=settings.secret_key,