feat(i18n): every server refusal is said by a record the contract holds

A refusal code or reason with no sentence fell through to the generic
"couldn't carry out that action" unnoticed: the parity test is green when
EN and HU miss the same key. One did (an edit refused for a file that is
not video or audio).

- lib/api/refusalCodes.ts: which sentence answers each code, as data;
  localizeDetail says a refusal through it, unchanged for every reader.
- backendIds contract: every detail code the backend sends is in exactly
  one record, every reason it can carry has its sentence in EN and HU,
  and no family names a reason the backend cannot send.
- The reasons that travel in a variable are Literal types (edit, move,
  cookies, quota), so the type checker refuses an unknown one too.
- downloads.errors.edit.not_media in EN and HU.

backend/app/reader/service.py

@@ -9,7 +9,7 @@ unique key, a row lock taken before a presence check. The comfort shelf's writes
 
 from datetime import datetime, timezone
 
-from typing import cast
+from typing import Literal, cast
 
 from sqlalchemy import Table, bindparam, delete, exists, func, select, text, update
 from sqlalchemy.dialects.postgresql import insert as pg_insert
@@ -301,10 +301,15 @@ def set_progress(db: Session, user: User, video_id: str, position: int) -> None:
 # --- playlists -------------------------------------------------------------------------------
 
 
+# Every reason a move is refused; the UI names each one (`playlists.moveErrors.<reason>`, held by the
+# frontend's backendIds contract, which reads this Literal).
+MoveReason = Literal["same_playlist", "not_in_playlist"]
+
+
 class MoveRefused(Exception):
     """A move between playlists that cannot happen: `reason` is the code the UI translates."""
 
-    def __init__(self, reason: str):
+    def __init__(self, reason: MoveReason):
         super().__init__(reason)
         self.reason = reason