fix(youtube): a refused YouTube action answers a 422 that says what to do

The two manual syncs (subscriptions import, playlist pull) had no YouTubeError
handler, so a quota, token or transport failure was a raw 500 and the blocking
"Server error" dialog (E9 F1). The other YouTube routes answered either the
upstream's English or a playlist-scoped "youtube_failed".

One helper, routes/_common.youtube_refusal: 422 {code: "youtube", reason} with
the reason sorted by what the user can do — quota (wait for the reset),
reconnect (the token step refused), unavailable (retry). Used by every route
that surfaces a YouTube failure; localizeDetail translates it.

"Reconnect" had no path: a grant Google revoked still read as granted, so
Settings and the wizard offered no connect button while every call failed.
An invalid_grant from the token endpoint now clears the stored grant.

backend/app/routes/feed.py

@@ -29,7 +29,7 @@ from app.models import (
     Video,
     VideoState,
 )
-from app.routes._common import get_or_404
+from app.routes._common import get_or_404, youtube_refusal
 from app.sync.videos import parse_iso8601_duration
 from app.utils import iso
 from app.youtube.client import YouTubeClient, YouTubeError
@@ -411,7 +411,7 @@ def get_video_detail(
         ):
             items = yt.get_videos([video_id])
     except YouTubeError as exc:
-        raise HTTPException(status_code=422, detail=f"YouTube lookup failed: {exc}")
+        raise youtube_refusal(exc)
     if not items:
         raise HTTPException(status_code=404, detail="Unknown video")