fix: invalidate index discographies after library rescan

The search index's Tier 3 (library artists) depends on MBIDs from
the artists table. If the index built before a rescan populated
those MBIDs, library artists like Flatbush Zombies wouldn't be
indexed — they're not in the sitewide top 1000 and their name
didn't match via fuzzy matching.

Fix: OnAllScansComplete hook now calls InvalidateIndexDiscographies
before StartIndexBuild. This clears the discog_built timestamp so
Tiers 2-4 re-run incrementally, picking up any new library artists
whose MBIDs were just populated by the scan.

The rebuild is incremental — only artists not already in the index
get their discographies fetched.

backend/app.go

@@ -218,6 +218,9 @@ func (yj *YellowJacketApp) OnStartup(ctx context.Context) {
 	yj.library.SetScanHooks(library.ScanHooks{
 		ResolvePhantoms: yj.playlist.ResolvePhantomTracksAfterScan,
 		OnAllScansComplete: func() {
+			// Force index Tiers 2-4 to re-check for new library
+			// artists whose MBIDs were just populated by the scan.
+			yj.explore.InvalidateIndexDiscographies()
 			yj.explore.StartIndexBuild()
 		},
 	})

backend/explore/explore.go

@@ -78,6 +78,13 @@ func (e *Service) StopIndexBuild() {
 	e.index.StopBuild()
 }
 
+// InvalidateIndexDiscographies clears the discography build
+// timestamp so the next index build re-runs Tiers 2-4.  Call
+// after a library rescan that may have populated new MBIDs.
+func (e *Service) InvalidateIndexDiscographies() {
+	e.index.InvalidateDiscographies()
+}
+
 // ---------------------------------------------------------------------------
 // MusicBrainz search
 // ---------------------------------------------------------------------------

backend/explore/searchindex.go

@@ -1381,6 +1381,14 @@ func filterUnindexed(artists []lbSitewideArtist, indexed map[string]bool) []lbSi
 	return out
 }
 
+// InvalidateDiscographies clears the discography build timestamp
+// so the next build re-runs Tiers 2-4.
+func (si *SearchIndex) InvalidateDiscographies() {
+	_, _ = si.db.ExecContext(
+		"DELETE FROM explore_index_meta WHERE key = 'discog_built'",
+	)
+}
+
 func (si *SearchIndex) setMeta(key, value string) {
 	if _, err := si.db.ExecContext(
 		"INSERT OR REPLACE INTO explore_index_meta (key, value) VALUES (?, ?)",