perf: unified per-artist indexing — discography + image in parallel

Restructure indexOneArtist to run LB discography fetches and MB
artist image resolution concurrently. They use different rate
limiters (LB: 3 req/s, MB: 1 req/s) so they overlap without
contention.

Per artist, the indexer now runs two parallel pipelines:
  LB pipeline: top-release-groups + top-recordings
  MB pipeline: url-rels → Wikidata P18 → Wikimedia image fetch

All artist images are pre-cached during the index build instead
of being resolved on-demand during search. Total build time
drops from ~105 min (sequential) to ~63 min (parallel, MB-bound).

SearchIndex now takes ArtistImageProvider as a dependency. The
Service constructor creates artistImg before the index so both
can share it.

backend/explore/explore.go

@@ -36,11 +36,11 @@ func NewExploreService(logger *slog.Logger, db *database.DB) *Service {
 	limiter := NewRateLimiter()
 	mb := NewMusicBrainzClient(cache, logger.WithGroup("musicbrainz"))
 	lb := NewListenBrainzClient(limiter, cache, logger.WithGroup("listenbrainz"))
-	index := NewSearchIndex(db, lb, logger.WithGroup("search-index"))
 	artProxy := NewCoverArtProxy(db, limiter)
 	artistImg := NewArtistImageProvider(
 		db, cache, NewRateLimiter(), logger.WithGroup("artist-image"),
 	)
+	index := NewSearchIndex(db, lb, artistImg, logger.WithGroup("search-index"))
 
 	logger.Info("explore service created")