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/library/scanner.go

@@ -70,7 +70,11 @@ var audioExtensions = map[string]bool{
 //	   release-track name, so every Vorbis-tagged track came in without its
 //	   tag id and waited on an AcoustID lookup. A re-read stores the tag id,
 //	   over a looked-up one (D4).
-const tagReadVersion int16 = 5
+//	6: a Vorbis TRACKNUMBER or DISCNUMBER written as "3/20" is read (#6171).
+//	   dhowden/tag reads it as 0, and Minstrel's own importer wrote it that
+//	   way, so all 245 FLAC tracks it imported came in with no track or disc
+//	   number and their albums lost their order.
+const tagReadVersion int16 = 6
 
 type Stats struct {
 	Scanned int `json:"scanned"`
@@ -316,8 +320,7 @@ func (s *Scanner) scanFile(
 		return pgtype.UUID{}, false, fmt.Errorf("album: %w", err)
 	}
 
-	trackNum, _ := meta.Track()
-	discNum, _ := meta.Disc()
+	trackNum, discNum := ReadTrackAndDisc(meta)
 
 	// An unchanged file being re-read only to refresh tag-derived columns
 	// doesn't need another ffprobe: the stored duration is still accurate, and