perf: burst-friendly MB rate limiter for interactive search

The shared 1 req/sec MB rate limiter was serializing the 3 concurrent
search calls in Phase 1 to ~3s minimum. Interactive search needs short
bursts (3 calls at once) but not sustained throughput.

Split into two MB rate limiters:
- mbSearchLimiter: burst=3, refill=1/sec — allows one search's 3
  concurrent calls to fire immediately, then rate-limits sustained use
- mbBackgroundLimiter: strict 1/sec — gates artist image resolution
  in the indexer to avoid 429s during sustained background work

Added NewRateLimiterBurst(n, b) constructor for configurable burst.

Expected Phase 1 improvement: ~3.5s → ~1s (3 calls fire in parallel
instead of serializing through the limiter).

backend/explore/explore.go

@@ -36,12 +36,16 @@ type Service struct {
 func NewExploreService(logger *slog.Logger, db *database.DB) *Service {
 	cache := NewCache(db, logger.WithGroup("cache"))
 	lbLimiter := NewRateLimiter()
-	mbLimiter := NewRateLimiter() // 1 req/sec, shared across all MB consumers
+	// MB search limiter: burst of 3 (covers one search's 3 concurrent calls)
-	mb := NewMusicBrainzClient(cache, mbLimiter, logger.WithGroup("musicbrainz"))
+	// then 1/sec refill.  The musicbrainzws2 library retries on 429 as backup.
+	mbSearchLimiter := NewRateLimiterBurst(1, 3)
+	// MB background limiter: strict 1/sec for sustained image resolution calls.
+	mbBackgroundLimiter := NewRateLimiter()
+	mb := NewMusicBrainzClient(cache, mbSearchLimiter, logger.WithGroup("musicbrainz"))
 	lb := NewListenBrainzClient(lbLimiter, cache, logger.WithGroup("listenbrainz"))
 	artProxy := NewCoverArtProxy(db, lbLimiter)
 	artistImg := NewArtistImageProvider(
-		db, cache, mbLimiter, logger.WithGroup("artist-image"),
+		db, cache, mbBackgroundLimiter, logger.WithGroup("artist-image"),
 	)
 	index := NewSearchIndex(db, lb, artistImg, logger.WithGroup("search-index"))
 	libMBID := NewLibraryMBIDIndex(db)

backend/explore/ratelimiter.go

@@ -46,6 +46,16 @@ func NewRateLimiterF(f float64) *RateLimiter {
 	}
 }
 
+// NewRateLimiterBurst returns a rate limiter that allows n requests
+// per second with a burst size of b.  The burst allows short spikes
+// (e.g. 3 concurrent search calls) without queueing, while still
+// limiting sustained throughput.
+func NewRateLimiterBurst(n, b int) *RateLimiter {
+	return &RateLimiter{
+		limiter: rate.NewLimiter(rate.Limit(n), b),
+	}
+}
+
 // Wait blocks until the rate limiter allows the caller to proceed
 // or the context is cancelled. Returns ctx.Err() if the context
 // expires before a token becomes available.