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/_common.py

@@ -13,6 +13,7 @@ from sqlalchemy.orm import Session
 
 from app.db import Base
 from app.models import User
+from app.youtube.client import YouTubeError, refusal_reason
 
 M = TypeVar("M", bound=Base)
 
@@ -51,3 +52,12 @@ def owned_or_404(
     if getattr(obj, owner_attr) != user.id:
         raise not_found(what)
     return obj
+
+
+def youtube_refusal(exc: YouTubeError) -> 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)})