perf: lightweight post-scan indexing — only index new artists

After a library scan completes, OnAllScansComplete now calls
IndexNewArtists() instead of StartIndexBuild(). This skips the
full tier pipeline (sitewide top artists, similar artists, freshness
checks) and only indexes library artists whose MBIDs are not yet
in the search index.

Flow after scan:
1. Query indexed artist MBIDs (fast, in-memory set)
2. Query library artist MBIDs
3. Diff → only new artists
4. Fetch discographies + images for new artists only

The full StartIndexBuild() still runs on initial launch (when
SoftScanAllLibraries finds no work to do) to handle the tier
pipeline with freshness-based refresh. But adding 5 new albums
to your library no longer triggers a 60-minute index rebuild.

backend/app.go

@@ -232,13 +232,9 @@ func (yj *YellowJacketApp) OnStartup(ctx context.Context) {
 	yj.library.SetScanHooks(library.ScanHooks{
 		ResolvePhantoms: yj.playlist.ResolvePhantomTracksAfterScan,
 		OnAllScansComplete: func() {
-			// Start the index build after scans finish.
-			// Tiers 2-4 are incremental — filterUnindexed
-			// already skips artists that are already indexed,
-			// so new library artists with freshly-populated
-			// MBIDs get picked up without invalidating the
-			// entire discography cache.
-			yj.explore.StartIndexBuild()
+			// Only index artists that are new since the last
+			// build — don't re-run the full tier pipeline.
+			yj.explore.IndexNewArtists()
 		},
 	})
 

backend/explore/explore.go

@@ -72,6 +72,12 @@ func (e *Service) StartIndexBuild() {
 	e.index.StartBuild(e.ctx)
 }
 
+// IndexNewArtists indexes only library artists not yet in the search
+// index. Lightweight post-scan path — skips the full tier machinery.
+func (e *Service) IndexNewArtists() {
+	e.index.IndexNewArtists(e.ctx)
+}
+
 // StopIndexBuild cancels the background search index build.
 // Call before a full rescan to free the DB for the scan.
 func (e *Service) StopIndexBuild() {

backend/explore/searchindex.go

@@ -134,6 +134,103 @@ func NewSearchIndex(
 	}
 }
 
+// IndexNewArtists indexes only library artists that are not yet in the
+// search index. This is the lightweight post-scan path — no tier
+// machinery, no freshness checks, no sitewide/similar artist logic.
+// Just finds library artists with MBIDs missing from the index and
+// fetches their discographies + images.
+func (si *SearchIndex) IndexNewArtists(ctx context.Context) {
+	si.mu.Lock()
+	if si.cancel != nil {
+		// Full build already running — it will pick up new artists.
+		si.mu.Unlock()
+
+		return
+	}
+
+	si.done = make(chan struct{})
+	si.mu.Unlock()
+
+	buildCtx, cancel := context.WithCancel(ctx)
+
+	si.mu.Lock()
+	si.cancel = cancel
+	si.mu.Unlock()
+
+	go func() {
+		defer func() {
+			si.mu.Lock()
+			si.cancel = nil
+			si.mu.Unlock()
+
+			close(si.done)
+		}()
+
+		si.indexNewLibraryArtists(buildCtx)
+	}()
+}
+
+// indexNewLibraryArtists finds library artists with MBIDs that are not
+// in the index and fetches their discographies.
+func (si *SearchIndex) indexNewLibraryArtists(ctx context.Context) {
+	indexed := si.indexedArtistMBIDs()
+	libraryMBIDs := si.getLibraryArtistMBIDs()
+
+	var newArtists []lbSitewideArtist
+
+	for _, mbid := range libraryMBIDs {
+		if !indexed[mbid] {
+			// Look up the artist name from the DB.
+			var name string
+
+			rows, err := si.db.QueryContext(
+				"SELECT name FROM artists WHERE mbid = ? LIMIT 1", mbid,
+			)
+			if err != nil {
+				continue
+			}
+
+			if !rows.Next() {
+				_ = rows.Close()
+
+				continue
+			}
+
+			if err := rows.Scan(&name); err != nil {
+				_ = rows.Close()
+
+				continue
+			}
+
+			_ = rows.Close()
+
+			newArtists = append(newArtists, lbSitewideArtist{
+				ArtistMBID: mbid,
+				ArtistName: name,
+			})
+		}
+	}
+
+	if len(newArtists) == 0 {
+		si.logger.Info("search index: no new library artists to index")
+
+		return
+	}
+
+	si.logger.Info("search index: indexing new library artists",
+		"count", len(newArtists),
+	)
+
+	indexLimiter := NewRateLimiterN(indexerRate)
+	indexLB := NewListenBrainzClient(indexLimiter, si.lb.cache, si.logger.WithGroup("indexer"))
+
+	si.indexArtistDiscographies(ctx, indexLB, newArtists, "new-artists")
+
+	si.logger.Info("search index: new library artists indexed",
+		"count", len(newArtists),
+	)
+}
+
 // StartBuild launches the background index build goroutine.
 // Returns immediately.
 func (si *SearchIndex) StartBuild(ctx context.Context) {