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/routes/plex/playback.py

@@ -10,6 +10,7 @@ from fastapi import (
     BackgroundTasks,
     Depends,
     HTTPException,
+    Path,
     Query,
     Request,
     Response,
@@ -95,12 +96,19 @@ def image(rating_key: str, request: Request, variant: str = "thumb") -> Response
 # poster, not for a route that starts reads of the media library: a suspended or "log out
 # everywhere"-revoked cookie kept scheduling them. They are not bursty (one poll a second, one fetch
 # per track), so the DB user-load costs nothing that matters.
+#
+# Both path parameters are bounded, the same way on both routes: they become a cache FILE name
+# (`sub_<rating_key>_<ord>`), and an absurd one — a 250-digit key or ordinal — outgrew the OS's name
+# limit and turned the status poll into a 500. A Plex rating_key is a short number; the bound only
+# has to be generous.
+_RatingKey = Path(max_length=32)
+_SubOrd = Path(ge=0, le=9999)
 
 
 @router.get("/subtitle/{rating_key}/{sub_ord}/status")
 def subtitle_status(
-    rating_key: str,
-    sub_ord: int,
+    rating_key: str = _RatingKey,
+    sub_ord: int = _SubOrd,
     user: User = Depends(current_user),
     # Bounded both ways: a non-finite position used to reach the scheduler's arithmetic and 500, and
     # an absurd one reached ffmpeg's `-ss`, which refused it.
@@ -117,8 +125,8 @@ def subtitle_status(
 
 @router.get("/subtitle/{rating_key}/{sub_ord}")
 def subtitle(
-    rating_key: str,
-    sub_ord: int,
+    rating_key: str = _RatingKey,
+    sub_ord: int = _SubOrd,
     user: User = Depends(current_user),
     # Finite, like every float this router takes: `-inf` crashed the shift (a 500), `nan` stacked
     # every cue at 0. It can legitimately be negative (a subtitle nudged later than the session start).