fix(13-02): resolve phantom playlist tracks using M3U8 paths after scan

- Add ScanHooks callback struct to library package (follows RemovalHooks pattern)
- Move phantom resolution from library to playlist service via hook
- New ResolvePhantomTracksAfterScan reads M3U8 files and resolves paths
  against current audio_files using multi-root resolution
- Handles pre-existing phantoms (match by M3U8 position) and new ones
  (match by phantom_file_path)
- Delete old resolvePhantomTracks method that required phantom_file_path
- Wire ScanHooks in app.go OnStartup

backend/app.go

@@ -171,6 +171,12 @@ func (yj *YellowJacketApp) OnStartup(ctx context.Context) {
 		PostScan: yj.playlist.RestoreAllPlaylists,
 	})
 
+	// Wire scan hooks so the playlist service can resolve
+	// phantom tracks after each library scan completes.
+	yj.library.SetScanHooks(library.ScanHooks{
+		ResolvePhantoms: yj.playlist.ResolvePhantomTracksAfterScan,
+	})
+
 	// Wire removal hooks so the library can stop playback and
 	// compact the queue during library removal without depending
 	// on the player or queue packages directly.