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.

frontend/src/i18n/locales/en/errors.json

@@ -13,7 +13,8 @@
     "move_target": "The playlist you're moving it to doesn't exist any more.",
     "video": "This video isn't in the library any more.",
     "invite": "This invite doesn't exist any more.",
-    "user": "This user doesn't exist any more."
+    "user": "This user doesn't exist any more.",
+    "channel": "This channel isn't in the library any more."
   },
   "ok": "OK",
   "offline": {

frontend/src/i18n/locales/hu/errors.json

@@ -13,7 +13,8 @@
     "move_target": "A lejátszási lista, ahová áthelyeznéd, már nem létezik.",
     "video": "Ez a videó már nincs a könyvtárban.",
     "invite": "Ez a meghívó már nem létezik.",
-    "user": "Ez a felhasználó már nem létezik."
+    "user": "Ez a felhasználó már nem létezik.",
+    "channel": "Ez a csatorna már nincs a könyvtárban."
   },
   "ok": "OK",
   "offline": {