perf: optimize index build — permanent caches + faster MB rate limit

Three changes to reduce subsequent index build times:

1. Positive image/rels cache TTL: 30 days → 365 days
   Artist url-rels and resolved image URLs rarely change.
   Already-indexed artists make zero API calls on rebuild.

2. Negative cache (misses) stays at 30 days so new images
   are discovered within a month of being added upstream.

3. MB rate limiter for background indexing: 1.0 → 1.5 req/s
   url-rels lookups are lightweight; 1.5/s is well within
   what MB handles (Picard and Kodi both use similar rates).
   Cuts the MB-bound portion of index build by ~33%.

Also adds NewRateLimiterF for fractional rates and caches
MB rels fetch failures (30-day miss TTL) to avoid retrying
unreachable artists every build.

backend/explore/explore.go

@@ -39,7 +39,7 @@ func NewExploreService(logger *slog.Logger, db *database.DB) *Service {
 	lb := NewListenBrainzClient(limiter, cache, logger.WithGroup("listenbrainz"))
 	artProxy := NewCoverArtProxy(db, limiter)
 	artistImg := NewArtistImageProvider(
-		db, cache, NewRateLimiter(), logger.WithGroup("artist-image"),
+		db, cache, NewRateLimiterF(1.5), logger.WithGroup("artist-image"),
 	)
 	index := NewSearchIndex(db, lb, artistImg, logger.WithGroup("search-index"))
 	libMBID := NewLibraryMBIDIndex(db)