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/refusal.py

@@ -8,8 +8,10 @@ whatever sentence the route happened to hand-write. `refused` is the generic fam
 (`not_found`, `youtube`, `playlist`, the admin refusals, …) stay, since each is already translated.
 
 A test (`test_http_details.py`) reads every `HTTPException` in the app and fails on a string detail,
-and the frontend's contract test requires every `refused(…, "<reason>")` to be named in both
-languages — so the rule is enforced, not described.
+and the frontend's contract test (`i18n/backendIds.contract.test.ts`) requires every `code` a detail
+carries to be in one record of `lib/api/refusalCodes.ts`, and every reason it can be sent with — a
+literal, or a `Literal` type when it travels in a variable — to have its sentence in both languages.
+So the rule is enforced, not described.
 """
 
 from fastapi import HTTPException