Adds Last.fm as a hybrid MBID-or-name provider in the artist + album
chains. Default-disabled; activates when the operator pastes a free
API key into the admin Cover Art panel. Rate-limited to 4 req/s under
their 5/s per-key ceiling.
Prefers MBID lookup when present (artist.getInfo accepts &mbid=),
falls back to artist-name search. For albums, requires both
ArtistName and AlbumTitle; MBID is sent as an additional hint.
Includes placeholder-image detection: Last.fm returns a generic
"default star" image hash for many artists rather than 404'ing.
Provider parses the response URL's basename hash and treats known
placeholders as ErrNotFound so we never persist a placeholder thumb
as real art. Fail-open: when in doubt, return ErrNotFound rather than
risk persisting wrong art.
When API key is empty, every call returns ErrNotFound regardless of
the enabled flag — defensive so an operator who toggles enabled
without setting a key doesn't see confusing transient errors.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
bvandeusen