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/mbids_test.go

@@ -242,3 +242,22 @@ func TestReadTagMBIDs_Recording(t *testing.T) {
 		t.Errorf("release track only: recording = %q, want empty", got)
 	}
 }
+
+// A Vorbis TRACKNUMBER of "3/20", as Minstrel's importer wrote it, is track 3
+// (#6171): dhowden/tag reads it as 0.
+func TestReadTrackAndDisc(t *testing.T) {
+	for _, tc := range []struct {
+		name        string
+		m           stubMeta
+		track, disc int
+	}{
+		{"vorbis with counts", stubMeta{format: tag.VORBIS, raw: map[string]interface{}{"tracknumber": "3/20", "discnumber": "2/2"}}, 3, 2},
+		{"vorbis bare", stubMeta{format: tag.VORBIS, raw: map[string]interface{}{"tracknumber": " 7 "}}, 7, 0},
+		{"vorbis junk", stubMeta{format: tag.VORBIS, raw: map[string]interface{}{"tracknumber": "A1", "discnumber": "-1"}}, 0, 0},
+		{"id3 is the library's own reading", stubMeta{format: tag.ID3v2_4, raw: map[string]interface{}{"tracknumber": "3/20"}}, 0, 0},
+	} {
+		if track, disc := ReadTrackAndDisc(tc.m); track != tc.track || disc != tc.disc {
+			t.Errorf("%s: got track %d disc %d, want %d and %d", tc.name, track, disc, tc.track, tc.disc)
+		}
+	}
+}

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

internal/library/tracknumber.go

@@ -0,0 +1,40 @@
+package library
+
+import (
+	"strconv"
+	"strings"
+
+	"github.com/dhowden/tag"
+)
+
+// ReadTrackAndDisc reads a file's track and disc numbers. dhowden/tag reads a
+// Vorbis TRACKNUMBER or DISCNUMBER only as a bare number, but taggers also
+// write "3/20" there (Minstrel's own importer did until #6171), and that
+// reads as 0. For Vorbis the number before the slash is taken instead.
+func ReadTrackAndDisc(m tag.Metadata) (track, disc int) {
+	track, _ = m.Track()
+	disc, _ = m.Disc()
+	if m.Format() != tag.VORBIS {
+		return track, disc
+	}
+	raw := m.Raw()
+	if track == 0 {
+		track = leadingNumber(raw["tracknumber"])
+	}
+	if disc == 0 {
+		disc = leadingNumber(raw["discnumber"])
+	}
+	return track, disc
+}
+
+// leadingNumber is the number in a tag value of "3" or "3/20"; 0 for
+// anything else.
+func leadingNumber(v any) int {
+	s, _ := v.(string)
+	before, _, _ := strings.Cut(s, "/")
+	n, err := strconv.Atoi(strings.TrimSpace(before))
+	if err != nil || n < 0 {
+		return 0
+	}
+	return n
+}