feat(subsonic): expose replaygain tags

tags/tagcommon/tagcommmon.go

@@ -24,9 +24,15 @@ type Info interface {
 	Genres() []string
 	TrackNumber() int
 	DiscNumber() int
+	Year() int
+
+	ReplayGainTrackGain() float32
+	ReplayGainTrackPeak() float32
+	ReplayGainAlbumGain() float32
+	ReplayGainAlbumPeak() float32
+
 	Length() int
 	Bitrate() int
-	Year() int
 }
 
 const (

tags/taglib/taglib.go

@@ -51,8 +51,14 @@ func (i *info) Genres() []string       { return find(i.raw, "genres") }
 func (i *info) TrackNumber() int       { return intSep("/", first(find(i.raw, "tracknumber"))) }                  // eg. 5/12
 func (i *info) DiscNumber() int        { return intSep("/", first(find(i.raw, "discnumber"))) }                   // eg. 1/2
 func (i *info) Year() int              { return intSep("-", first(find(i.raw, "originaldate", "date", "year"))) } // eg. 2023-12-01
-func (i *info) Length() int            { return i.props.Length }
-func (i *info) Bitrate() int           { return i.props.Bitrate }
+
+func (i *info) ReplayGainTrackGain() float32 { return dB(first(find(i.raw, "replaygain_track_gain"))) }
+func (i *info) ReplayGainTrackPeak() float32 { return flt(first(find(i.raw, "replaygain_track_peak"))) }
+func (i *info) ReplayGainAlbumGain() float32 { return dB(first(find(i.raw, "replaygain_album_gain"))) }
+func (i *info) ReplayGainAlbumPeak() float32 { return flt(first(find(i.raw, "replaygain_album_peak"))) }
+
+func (i *info) Length() int  { return i.props.Length }
+func (i *info) Bitrate() int { return i.props.Bitrate }
 
 func first[T comparable](is []T) T {
 	var z T
@@ -83,6 +89,17 @@ func filterStr(ss []string) []string {
 	return r
 }
 
+func flt(in string) float32 {
+	f, _ := strconv.ParseFloat(in, 32)
+	return float32(f)
+}
+func dB(in string) float32 {
+	in = strings.ToLower(in)
+	in = strings.TrimSuffix(in, " db")
+	in = strings.TrimSuffix(in, "db")
+	return flt(in)
+}
+
 func intSep(sep, in string) int {
 	start, _, _ := strings.Cut(in, sep)
 	out, _ := strconv.Atoi(start)