Three bugs causing all artist images to be the same Last.fm placeholder:
1. Last.fm stopped serving real artist images years ago and returns a
generic placeholder (hash 2a96cbd8b46e442fc41c2b86b821562f) for
every artist. Added filter in selectBestImage to reject URLs
containing this known placeholder hash.
2. Provider order had Last.fm before Deezer for artist images. Since
Last.fm "succeeded" with the placeholder, Deezer was never reached.
Swapped order: Deezer now checked before Last.fm.
3. FetchMissingArtistImages had inverted if/else — aliases were used
on error, bare name on success. Fixed condition to err == nil.
