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/playlists.py

@@ -23,7 +23,7 @@ from app.sync.playlists import (
     sync_user_playlists,
 )
 from app.ratelimit import RateLimiter
-from app.routes._common import get_or_404, owned_or_404
+from app.routes._common import get_or_404, owned_or_404, youtube_refusal
 from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api/playlists", tags=["playlists"])
@@ -322,8 +322,12 @@ def sync_youtube(
     if not has_read_scope(user):
         raise _refused(403, "youtube_read_scope")
     _check_pull_cooldown(user)
-    with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
-        return sync_user_playlists(db, user)
+    try:
+        with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
+            return sync_user_playlists(db, user)
+    except YouTubeError as exc:
+        db.rollback()
+        raise youtube_refusal(exc)
 
 
 @router.post("/{playlist_id}/revert-youtube")
@@ -344,9 +348,9 @@ def revert_youtube(
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PULL):
             return repull_playlist(db, user, pl)
-    except YouTubeError:
+    except YouTubeError as exc:
         db.rollback()
-        raise _refused(422, "youtube_failed")
+        raise youtube_refusal(exc)
 
 
 def _pushable(db: Session, user: User, playlist_id: int) -> Playlist:
@@ -371,8 +375,8 @@ def push_plan(
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
             plan = plan_merge(db, user, pl)
-    except YouTubeError:
-        raise _refused(422, "youtube_failed")
+    except YouTubeError as exc:
+        raise youtube_refusal(exc)
     plan["remaining_today"] = quota.remaining_today(db)
     plan["affordable"] = plan["units_estimate"] <= plan["remaining_today"]
     return plan
@@ -404,9 +408,9 @@ def push(
             if plan["units_estimate"] > quota.remaining_today(db):
                 raise _refused(429, "quota_exceeded")
             return merge_playlist(db, user, pl, live=live)
-    except YouTubeError:
+    except YouTubeError as exc:
         db.rollback()
-        raise _refused(422, "youtube_failed")
+        raise youtube_refusal(exc)
 
 
 @router.get("/{playlist_id}")
@@ -496,8 +500,8 @@ def delete_playlist(
             with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_DELETE):
                 with YouTubeClient(db, user) as yt:
                     yt.delete_playlist(pl.yt_playlist_id)
-        except YouTubeError:
-            raise _refused(422, "youtube_failed")
+        except YouTubeError as exc:
+            raise youtube_refusal(exc)
     db.delete(pl)  # items cascade
     db.commit()
     return {"deleted": playlist_id}