fix(i18n): every refusal the API answers is structured and said in the reader's language

~150 HTTPExceptions carried a hand-written English sentence as their detail,
shown verbatim under the Hungarian UI — in the blocking error dialog and on
every inline surface that printed HttpError.detail (sign-in, setup wizard,
change password, the passphrase gate, live search). Four also quoted an
upstream exception's text back to the user.

- app/refusal.py: `refused(status, reason, **params)` → {code: "refused",
  reason, …params}; localizeDetail translates errors.refused.<reason> with the
  params interpolated. "Unknown X" 404s use the existing not_found(what).
- sysconfig.set_value raises InvalidSetting (reason + bounds) instead of an
  English ValueError; the three writers answer it through setting_refused.
- The Plex connection/write/image failures log what the upstream said and
  answer a reason; the search scraper's failure is logged and answered as
  youtube/unavailable (it crashed refusal_reason before).
- HttpError.detail now only ever holds a translated reason: a string detail
  (only the framework's own can remain) is left out, so reading .detail is
  safe everywhere — including destructured, the hole the raw-detail source
  scan could not see. That scan and its five-file exception list are gone.
- Enforced: test_http_details.py fails on any HTTPException with a sentence
  for a detail; the contract test requires every refused reason and every
  not_found "what" in both languages, and no key for one that is gone.
- local/jsx-comment-text: a // inside JSX children renders as text; the rule
  is ten lines in eslint.config.js rather than eslint-plugin-react's ~1800
  lockfile lines.

backend/app/routes/_common.py

@@ -46,18 +46,20 @@ def owned_or_404(
 ) -> M:
     """Fetch a row by primary key that `user` owns, or raise 404. A row owned by someone
     else 404s (NOT 403) on purpose — a 403 would confirm the id exists to a stranger.
-    Generalizes the near-identical per-route ownership helpers (e.g. plex's
-    `_own_playlist_or_404`, the saved-views owner check)."""
+    It replaced the near-identical per-route ownership helpers, and is called directly — a thin
+    `_own_x` wrapper around it is the ad-hoc shape CLAUDE.md's patterns table rules out."""
     obj = get_or_404(db, model, id_, what=what)
     if getattr(obj, owner_attr) != user.id:
         raise not_found(what)
     return obj
 
 
-def youtube_refusal(exc: YouTubeError) -> HTTPException:
+def youtube_refusal(exc: Exception) -> HTTPException:
     """The answer to an action YouTube refused: a 422 whose reason the UI translates
     (`errors.youtube.<reason>`). One definition, because the routes had grown three shapes of it —
     a bare English string quoting the upstream message, a playlist-scoped reason, and nothing at
     all (a raw 500 from the manual syncs). The upstream failure is already logged where it
-    happened (`youtube.api.error`)."""
-    return HTTPException(status_code=422, detail={"code": "youtube", "reason": refusal_reason(exc)})
+    happened (`youtube.api.error`). A failure that is not the Data API's (the search scraper's) is
+    YouTube being unavailable to us — the retryable answer."""
+    reason = refusal_reason(exc) if isinstance(exc, YouTubeError) else "unavailable"
+    return HTTPException(status_code=422, detail={"code": "youtube", "reason": reason})