New album-cover and artist-art source implementing
AlbumCoverProvider, ArtistArtProvider, and TestableProvider. Hits
the documented /album-mb.php and /artist-mb.php JSON endpoints by
MBID, then GETs the returned image URLs (CDN, key-less). 2 req/sec
rate limit per the upstream's documented free/test-key tier;
mutex+lastCall serialises ALL HTTP calls regardless of method so
JSON metadata fetches and image GETs share the same budget.
429 / 5xx triggers exponential backoff with jitter (250ms × 2^attempt
± 20%) up to 3 retries before surfacing ErrTransient. 401 / 403
surface as ErrTransient (auth issues are config errors, not "this
album has no art" — the row stays NULL across passes so the operator
sees pending count not decreasing as the diagnostic signal).
Artist art is atomic at the JSON metadata step (one round-trip
yields both URLs); the two image GETs are independent — partial
success returns whatever bytes landed and ErrNotFound only if
neither URL is present in the response.
Configure() falls back to the documented public test key (2) when
the operator's APIKey is empty, per the no-coercive-settings
principle. Test connection hits a stable popular release MBID
(Pink Floyd, "The Dark Side of the Moon").
theAudioDBBaseURL is a var (not const) so tests can override it.
bvandeusen
8f64bcdb5f