fix: shared MB rate limiter prevents search/indexer 429 collisions

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.

backend/explore/explore.go

@@ -35,12 +35,13 @@ type Service struct {
 // client, and ListenBrainz client internally.
 func NewExploreService(logger *slog.Logger, db *database.DB) *Service {
 	cache := NewCache(db, logger.WithGroup("cache"))
-	limiter := NewRateLimiter()
-	mb := NewMusicBrainzClient(cache, logger.WithGroup("musicbrainz"))
-	lb := NewListenBrainzClient(limiter, cache, logger.WithGroup("listenbrainz"))
-	artProxy := NewCoverArtProxy(db, limiter)
+	lbLimiter := NewRateLimiter()
+	mbLimiter := NewRateLimiter() // 1 req/sec, shared across all MB consumers
+	mb := NewMusicBrainzClient(cache, mbLimiter, logger.WithGroup("musicbrainz"))
+	lb := NewListenBrainzClient(lbLimiter, cache, logger.WithGroup("listenbrainz"))
+	artProxy := NewCoverArtProxy(db, lbLimiter)
 	artistImg := NewArtistImageProvider(
-		db, cache, NewRateLimiterF(1.5), logger.WithGroup("artist-image"),
+		db, cache, mbLimiter, logger.WithGroup("artist-image"),
 	)
 	index := NewSearchIndex(db, lb, artistImg, logger.WithGroup("search-index"))
 	libMBID := NewLibraryMBIDIndex(db)

backend/explore/musicbrainz.go

@@ -23,18 +23,22 @@ const (
 // response cache.  Every API call checks the cache first and stores
 // successful responses for future hits.
 //
-// The underlying musicbrainzws2.Client handles MusicBrainz-specific
-// rate limiting via retries on HTTP 429, so we do not use the
-// RateLimiter from this package (that is reserved for ListenBrainz).
+// A proactive rate limiter gates all outgoing requests at 1 req/sec
+// to avoid triggering MusicBrainz 429 responses.  The underlying
+// musicbrainzws2.Client still retries on 429 as a safety net, but
+// the limiter should prevent most rate-limit hits.
 type MusicBrainzClient struct {
-	mb     *musicbrainzws2.Client
-	cache  *Cache
-	logger *slog.Logger
+	mb      *musicbrainzws2.Client
+	cache   *Cache
+	limiter *RateLimiter
+	logger  *slog.Logger
 }
 
 // NewMusicBrainzClient creates a MusicBrainz API client that caches
-// responses in the given Cache.
-func NewMusicBrainzClient(cache *Cache, logger *slog.Logger) *MusicBrainzClient {
+// responses in the given Cache.  The provided rate limiter is shared
+// with all other MB consumers (e.g. artist image resolution) to
+// prevent concurrent bursts from triggering 429s.
+func NewMusicBrainzClient(cache *Cache, limiter *RateLimiter, logger *slog.Logger) *MusicBrainzClient {
 	mb := musicbrainzws2.NewClient(musicbrainzws2.AppInfo{
 		Name:    "YellowJacket",
 		Version: "dev",
@@ -42,9 +46,10 @@ func NewMusicBrainzClient(cache *Cache, logger *slog.Logger) *MusicBrainzClient
 	})
 
 	return &MusicBrainzClient{
-		mb:     mb,
-		cache:  cache,
-		logger: logger,
+		mb:      mb,
+		cache:   cache,
+		limiter: limiter,
+		logger:  logger,
 	}
 }
 
@@ -71,6 +76,10 @@ func (c *MusicBrainzClient) SearchArtists(
 		}
 	}
 
+	if err := c.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+
 	c.logger.Info("musicbrainz search artists",
 		"query", query,
 		"limit", limit,
@@ -105,6 +114,10 @@ func (c *MusicBrainzClient) SearchReleaseGroups(
 		}
 	}
 
+	if err := c.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+
 	c.logger.Info("musicbrainz search release groups",
 		"query", query,
 		"limit", limit,
@@ -139,6 +152,10 @@ func (c *MusicBrainzClient) SearchRecordings(
 		}
 	}
 
+	if err := c.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+
 	c.logger.Info("musicbrainz search recordings",
 		"query", query,
 		"limit", limit,
@@ -176,6 +193,10 @@ func (c *MusicBrainzClient) LookupArtist(
 		}
 	}
 
+	if err := c.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+
 	c.logger.Info("musicbrainz lookup artist", "mbid", mbid)
 
 	a, err := c.mb.LookupArtist(ctx,
@@ -206,6 +227,10 @@ func (c *MusicBrainzClient) LookupReleaseGroup(
 		}
 	}
 
+	if err := c.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+
 	c.logger.Info("musicbrainz lookup release group", "mbid", mbid)
 
 	rg, err := c.mb.LookupReleaseGroup(ctx,
@@ -241,6 +266,10 @@ func (c *MusicBrainzClient) BrowseReleaseGroups(
 		}
 	}
 
+	if err := c.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+
 	c.logger.Info("musicbrainz browse release groups",
 		"artistMBID", artistMBID,
 	)
@@ -276,6 +305,10 @@ func (c *MusicBrainzClient) BrowseReleases(
 		}
 	}
 
+	if err := c.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+
 	c.logger.Info("musicbrainz browse releases",
 		"releaseGroupMBID", releaseGroupMBID,
 	)

frontend/wailsjs/go/explore/Service.d.ts

@@ -23,6 +23,8 @@ export function GetThumbnail(arg1:string,arg2:string,arg3:string):Promise<string
 
 export function GetThumbnails(arg1:Array<explore.ThumbnailRequest>):Promise<Record<string, string>>;
 
+export function IndexNewArtists():Promise<void>;
+
 export function InvalidateIndexDiscographies():Promise<void>;
 
 export function LookupArtist(arg1:string):Promise<explore.MBArtist>;

frontend/wailsjs/go/explore/Service.js

@@ -42,6 +42,10 @@ export function GetThumbnails(arg1) {
   return window['go']['explore']['Service']['GetThumbnails'](arg1);
 }
 
+export function IndexNewArtists() {
+  return window['go']['explore']['Service']['IndexNewArtists']();
+}
+
 export function InvalidateIndexDiscographies() {
   return window['go']['explore']['Service']['InvalidateIndexDiscographies']();
 }