fix: imported FLAC, Ogg and Opus files keep their track and disc numbers (#6171)

The importer wrote track=N/M for every format, which ffmpeg stores in a
Vorbis comment as TRACKNUMBER=3/20; dhowden/tag reads a Vorbis TRACKNUMBER
with a bare Atoi, so it came back 0. All 245 tracks imported since
2026-10-10 (Humanz, Plastic Beach, the Gorillaz super deluxe and twelve
more albums) have no track or disc number, and their album pages have no
order.

- The importer writes a Vorbis track and disc number alone, with
  TRACKTOTAL and DISCTOTAL, as Picard does. ID3 and MP4 keep N/M.
- ReadTrackAndDisc takes the number before a slash when the Vorbis reader
  returns 0, for the scanner and for the import's own check of a download.
- tagReadVersion 6 re-reads the files already held (tag reads only).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/importer/inspect.go

@@ -133,8 +133,7 @@ func readFile(ctx context.Context, root, path string) File {
 		return f
 	}
 	f.Title, f.Artist, f.AlbumArtist, f.Album = m.Title(), m.Artist(), m.AlbumArtist(), m.Album()
-	f.Track, _ = m.Track()
-	f.Disc, _ = m.Disc()
+	f.Track, f.Disc = library.ReadTrackAndDisc(m)
 	ids := library.ReadTagMBIDs(m)
 	f.GroupMBID, f.ReleaseMBID, f.RecordingMBID = ids.ReleaseGroup, ids.Release, ids.Recording
 	return f