fix(playlists): a racing move is not a 500; ownership through owned_or_404 everywhere

move_item checked "already in the target?" and then re-parented without a
lock, so a second tab's add or the scheduler's YouTube merge committing the
same video in between hit uq_playlist_video and answered a raw 500. The
re-parent now runs in a savepoint and a conflict falls back to the
already-present outcome. The target is touched only when the row landed in
it (an untouched list was promoted in the rail's "recently changed" sort),
and _touch stamps the database clock like the mixin's own onupdate.

The canonical-patterns table names owned_or_404 against ad-hoc ownership
helpers; six were left (_own_playlist, _own_playlist_or_404, _own_view,
_own_tag, notifications._owned, downloads._own_job), two of them
hand-rolled with a bare English 404. All call sites now use owned_or_404,
and the download/link/notification 404s are structured like the rest. The
contract test reads every "what is gone" the routes name, and the YouTube
refusal reasons, and requires both languages to name each.

backend/app/routes/notifications.py

@@ -5,7 +5,7 @@ All endpoints use `current_user` (not `require_human`): notifications and their
 state are personal UI state with no quota or YouTube involvement, so the shared demo account
 manages its own inbox normally."""
 
-from fastapi import APIRouter, Depends, HTTPException, Query
+from fastapi import APIRouter, Depends, Query
 from sqlalchemy import func, select, update
 from sqlalchemy.orm import Session
 
@@ -14,6 +14,7 @@ from app.db import get_db
 from app.models import Notification, User
 from app.notifications import trim_read
 from app.utils import iso
+from app.routes._common import owned_or_404
 
 router = APIRouter(prefix="/api/me/notifications", tags=["notifications"])
 
@@ -72,20 +73,13 @@ def unread_count(user: User = Depends(current_user), db: Session = Depends(get_d
     return {"count": count or 0}
 
 
-def _owned(db: Session, notification_id: int, user: User) -> Notification:
-    n = db.get(Notification, notification_id)
-    if n is None or n.user_id != user.id:
-        raise HTTPException(status_code=404, detail="Notification not found")
-    return n
-
-
 @router.post("/{notification_id}/read")
 def mark_read(
     notification_id: int,
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    n = _owned(db, notification_id, user)
+    n = owned_or_404(db, Notification, notification_id, user, what="notification")
     n.read = True
     db.commit()
     trim_read(db, user.id)
@@ -110,7 +104,7 @@ def dismiss(
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
 ) -> dict:
-    n = _owned(db, notification_id, user)
+    n = owned_or_404(db, Notification, notification_id, user, what="notification")
     # Dismiss implies read (it no longer counts toward the unread badge).
     n.dismissed = True
     n.read = True