persistArtist now calls repo.CopyAttributes(prevID, a.ID, "created_at")
after ReassignAnnotation, mirroring persistAlbum. Without this, every
PID.Artist config change reset the artist's created_at to scan time,
breaking 'recently added artists' views and any consumer that relies
on the original first-seen timestamp.
Adds CopyAttributes to ArtistRepository (mirroring AlbumRepository's
same-name method) with the same zero-poisoning guard for created_at.
363b93ef25