fix: mark search index ready at service creation, not just during build

The search index ready flag is an in-memory bool that resets to false
on every app restart. It was only set to true inside build(), which
runs in a goroutine after SoftScanAllLibraries completes. If the user
searched before the build goroutine started, IsReady() returned false
and the search took the slow path (LB popularity + cross-ref: ~2.3s)
even though the SQLite index had all the data from the previous build.

Now MarkReadyIfPopulated() is called eagerly in NewExploreService —
the index is queryable as soon as the service is constructed, before
any goroutines launch. If the explore_index table has rows, ready=true
immediately.

backend/explore/explore.go

@@ -48,6 +48,7 @@ func NewExploreService(logger *slog.Logger, db *database.DB) *Service {
 		db, cache, mbBackgroundLimiter, logger.WithGroup("artist-image"),
 	)
 	index := NewSearchIndex(db, lb, artistImg, logger.WithGroup("search-index"))
+	index.MarkReadyIfPopulated() // make index queryable immediately if data exists
 	libMBID := NewLibraryMBIDIndex(db)
 
 	logger.Info("explore service created")