CopyAttributes' cover-stamp guard read the source's uploaded_image only when
the caller also requested that column — a hidden cross-column contract a
future caller could silently violate. Fetch it internally instead, and align
the mock with the compound guard (it copied stamps from coverless sources).
Also cover the untested new code: Key() cover-stamp components in the disc
and mediafile readers, and the playlist tracks list path exposing
cover_art_updated_at.