When artists are purged during garbage collection, any custom uploaded
cover images were left orphaned on disk. Modified purgeEmpty() to query
for uploaded_image filenames before the bulk DELETE, then remove the
corresponding files from disk afterwards. Image cleanup is best-effort
to avoid failing the GC if a file is already missing or inaccessible.
Also populated album_artists entries in the persistence test suite setup
to reflect the actual album-artist relationships from test data, ensuring
purgeEmpty() doesn't inadvertently delete shared test artists.
Deluan
b013b71ba9