GetOrPlaceholder folded model.ErrNotFound in with ErrUnavailable, so an
id matching no entity returned 200 and the placeholder PNG. That made
the ErrorDataNotFound branch in getCoverArt unreachable: Subsonic went
from error 70 to a successful placeholder, and Jellyfin's Primary image
endpoint from 404 to 200. Neither is ours to change.
An entity with no art and an id with no entity are different answers;
only the first is a placeholder.