Previously, the MusicBrainzClient had no proactive rate limiter —
it relied on the musicbrainzws2 library's retry-on-429 backoff.
When the background indexer was resolving artist images (hitting MB
at 1.5 req/sec) and a user search fired 3+ concurrent MB calls,
the combined burst triggered 429s with cascading retries up to 60s.
Now a single shared RateLimiter (1 req/sec) gates all MB API calls:
- MusicBrainzClient search/lookup/browse methods
- ArtistImageProvider fetchMBRels (was on a separate 1.5 req/sec limiter)
The limiter serializes access proactively, preventing 429s entirely.
The musicbrainzws2 retry logic remains as a safety net.
Also split the old shared limiter into separate lbLimiter (for
ListenBrainz + CoverArt) and mbLimiter (for MusicBrainz) so the
two APIs don't block each other.
yonlu
a93a83c4d2