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")

backend/explore/searchindex.go

@@ -508,7 +508,7 @@ func (si *SearchIndex) build(ctx context.Context) {
 	si.logger.Info("search index build starting")
 
 	// Mark ready from existing rows so search works during the build.
-	si.markReadyIfPopulated()
+	si.MarkReadyIfPopulated()
 
 	indexLimiter := NewRateLimiterN(indexerRate)
 	indexLB := NewListenBrainzClient(indexLimiter, si.lb.cache, si.logger.WithGroup("indexer"))
@@ -1609,7 +1609,10 @@ func (si *SearchIndex) setMeta(key, value string) {
 	}
 }
 
-func (si *SearchIndex) markReadyIfPopulated() {
+// MarkReadyIfPopulated sets the index as ready for querying if it
+// already contains data from a previous build.  Called eagerly at
+// service creation so the index is queryable before StartBuild runs.
+func (si *SearchIndex) MarkReadyIfPopulated() {
 	rows, err := si.db.QueryContext("SELECT COUNT(*) FROM explore_index")
 	if err != nil {
 		return