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/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.