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/place.go

@@ -38,16 +38,32 @@ type Tags struct {
 // ID3 takes Picard's TXXX descriptions, which ffmpeg writes for any key it
 // has no frame for. MP4's freeform atoms ffmpeg cannot write, so an M4A
 // gets the plain names only.
+//
+// A Vorbis track or disc number is the number alone, its count in
+// TRACKTOTAL and DISCTOTAL, as Picard writes them: dhowden/tag reads
+// TRACKNUMBER "3/20" as 0, and every album imported before #6171 lost its
+// order that way. ID3 and MP4 take "3/20", which their readers parse.
 func (t Tags) args(ext string) []string {
+	vorbis := ext == ".flac" || ext == ".ogg" || ext == ".opus"
+	track, disc := ofN(t.Track, t.Tracks), ofN(t.Disc, t.Discs)
+	if vorbis {
+		track, disc = ofN(t.Track, 0), ofN(t.Disc, 0)
+	}
 	kv := [][2]string{
 		{"title", t.Title}, {"artist", t.Artist}, {"album", t.Album}, {"album_artist", t.AlbumArtist},
-		{"track", ofN(t.Track, t.Tracks)}, {"disc", ofN(t.Disc, t.Discs)},
+		{"track", track}, {"disc", disc},
 	}
 	if t.Year > 0 {
 		kv = append(kv, [2]string{"date", strconv.Itoa(t.Year)})
 	}
 	switch ext {
 	case ".flac", ".ogg", ".opus":
+		if t.Track > 0 && t.Tracks > 0 {
+			kv = append(kv, [2]string{"TRACKTOTAL", strconv.Itoa(t.Tracks)})
+		}
+		if t.Disc > 0 && t.Discs > 0 {
+			kv = append(kv, [2]string{"DISCTOTAL", strconv.Itoa(t.Discs)})
+		}
 		kv = append(kv,
 			[2]string{"MUSICBRAINZ_ALBUMID", t.ReleaseMBID},
 			[2]string{"MUSICBRAINZ_RELEASEGROUPID", t.GroupMBID},