fix(library): read every genre of FLAC, Ogg, Opus and MP4 files (#2500)

Vorbis comments repeat a field to give it several values (GENRE=Boom Bap,
GENRE=Downtempo, ...). dhowden/tag keeps comments in a map keyed by field
name, so each repeat overwrote the previous one and only the last genre
was stored. MP4 has the same gap: several data atoms in one ©gen atom, or
repeated ©gen atoms, collapse to one value.

On the operator's library 3,658 of 4,092 FLACs declare more than one
genre. Kupla's Life Forms carries eight and was stored as "Instrumental
Hip Hop" alone, so browse and the taste profile never saw the other seven.

- vorbisgenre.go reads the comment block directly: FLAC's metadata block
  (including FLACs behind an ID3v2 tag), and the comment packet of Ogg
  Vorbis and Opus, reassembled across pages when cover art makes it span
  several.
- mp4genre.go walks moov > udta > meta > ilst and returns every ©gen text
  value. It handles ISO and QuickTime meta layouts and a moov placed after
  mdat. A file with only the numeric gnre atom still falls back to
  dhowden, which resolves it.
- extractGenres routes VORBIS and MP4 through them, as #2499 did for
  ID3v2.
- tagReadVersion 3 -> 4, so the next scan re-reads the tags of files
  already indexed. Unchanged files keep their duration and fingerprint,
  so the pass costs tag reads only.

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

internal/library/genre.go

@@ -32,11 +32,25 @@ func extractGenres(meta tag.Metadata, rs io.ReadSeeker) (genres []string, fellBa
 		// No frame at all is the common case for untagged files, and
 		// dhowden/tag will have nothing either — not worth flagging.
 		fellBack = meta.Genre() != ""
-	default:
-		// Vorbis comments (FLAC/OGG/Opus) and MP4 atoms don't go through
-		// dhowden's welding path, so its value is already a faithful read of
-		// the primary genre. Multi-value handling for those containers is a
-		// separate, unproven concern — see #2500.
+	case tag.VORBIS, tag.MP4:
+		// dhowden/tag keeps only the last of a repeated field here (#2500):
+		// see vorbisgenre.go and mp4genre.go.
+		var values []string
+		var err error
+		switch {
+		case meta.Format() == tag.MP4:
+			values, err = readMP4GenreValues(rs)
+		case meta.FileType() == tag.OGG:
+			values, err = readOggGenreValues(rs)
+		default:
+			values, err = readFLACGenreValues(rs)
+		}
+		if err == nil && len(values) > 0 {
+			return normaliseGenres(values), false
+		}
+		// err == nil with no values: the file declares no genre of its own
+		// (or, for MP4, only the numeric gnre atom, which dhowden resolves).
+		fellBack = err != nil && meta.Genre() != ""
 	}
 	return normaliseGenres([]string{meta.Genre()}), fellBack
 }