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/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
+}