fix: stop invalidating discography cache after every library scan

Root cause: OnAllScansComplete called InvalidateIndexDiscographies()
which deleted the discog_built timestamp. SoftScanAllLibraries
triggers a scan whenever file counts differ (even by 1 file), so
on most launches the hook fired and wiped the cache.

The invalidation was unnecessary — Tiers 2-4 are already incremental.
filterUnindexed skips artists that are already indexed, so new
library artists with freshly-populated MBIDs get picked up naturally
without forcing a full rebuild.

InvalidateIndexDiscographies is kept as a public API for manual
rebuild (future UI button) but no longer called automatically.

Combined with per-tier timestamps from the previous commit, the
index build now correctly skips completed tiers on restart.

backend/app.go

@@ -232,9 +232,12 @@ 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
+			// Start the index build after scans finish.
-			// artists whose MBIDs were just populated by the scan.
+			// Tiers 2-4 are incremental — filterUnindexed
-			yj.explore.InvalidateIndexDiscographies()
+			// 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()
 		},
 	})