add format-aware tagging and replaygain fallback mapping

Write replaygain gain and peak tags from provider-specific metadata fields, then apply tagging across supported output containers with graceful cover-art fallback so non-FLAC downloads retain metadata.

internal/audio/tag/tagger.go

@@ -27,6 +27,8 @@ type Metadata struct {
 	ISRC                string
 	ReplaygainTrackGain string
 	ReplaygainAlbumGain string
+	ReplaygainTrackPeak string
+	ReplaygainAlbumPeak string
 	SourcePlatform      string
 	SourceTrackID       string
 	SourceAlbumID       string
@@ -44,11 +46,19 @@ func (t *Tagger) TagFLAC(path string, meta Metadata, coverPath string) error {
 		return fmt.Errorf("ffmpeg not found: %w", err)
 	}
 
-	tmpPath := path + ".tmp.flac"
-	args := buildFFmpegArgs(path, tmpPath, meta, coverPath)
+	ext := strings.TrimPrefix(strings.ToLower(filepath.Ext(path)), ".")
+	tmpPath := taggedTempPath(path)
+	runTag := func(cover string) ([]byte, error) {
+		args := buildFFmpegArgs(path, tmpPath, meta, cover, ext)
+		cmd := exec.Command("ffmpeg", args...)
+		return cmd.CombinedOutput()
+	}
 
-	cmd := exec.Command("ffmpeg", args...)
-	output, err := cmd.CombinedOutput()
+	output, err := runTag(coverPath)
+	if err != nil && strings.TrimSpace(coverPath) != "" {
+		_ = os.Remove(tmpPath)
+		output, err = runTag("")
+	}
 	if err != nil {
 		_ = os.Remove(tmpPath)
 		return fmt.Errorf("ffmpeg tag failed: %w: %s", err, string(output))
@@ -62,15 +72,16 @@ func (t *Tagger) TagFLAC(path string, meta Metadata, coverPath string) error {
 	return nil
 }
 
-func buildFFmpegArgs(inputPath, outputPath string, meta Metadata, coverPath string) []string {
+func buildFFmpegArgs(inputPath, outputPath string, meta Metadata, coverPath, ext string) []string {
 	args := []string{"-y", "-i", inputPath}
-	withCover := coverPath != "" && fileExists(coverPath)
+	withCover := coverPath != "" && fileExists(coverPath) && supportsAttachedPicture(ext)
 	if withCover {
 		args = append(args, "-i", coverPath)
 	}
 
 	args = append(args,
 		"-map", "0:a",
+		"-map_metadata", "0",
 		"-c:a", "copy",
 	)
 	if withCover {
@@ -79,6 +90,9 @@ func buildFFmpegArgs(inputPath, outputPath string, meta Metadata, coverPath stri
 			"-c:v", "mjpeg",
 			"-disposition:v:0", "attached_pic",
 		)
+		if ext == "mp3" {
+			args = append(args, "-id3v2_version", "3")
+		}
 	}
 
 	for k, v := range toTags(meta) {
@@ -92,6 +106,23 @@ func buildFFmpegArgs(inputPath, outputPath string, meta Metadata, coverPath stri
 	return args
 }
 
+func taggedTempPath(path string) string {
+	ext := filepath.Ext(path)
+	if ext == "" {
+		return path + ".tmp"
+	}
+	return strings.TrimSuffix(path, ext) + ".tmp" + ext
+}
+
+func supportsAttachedPicture(ext string) bool {
+	switch strings.TrimPrefix(strings.ToLower(ext), ".") {
+	case "flac", "mp3", "m4a", "mp4":
+		return true
+	default:
+		return false
+	}
+}
+
 func toTags(meta Metadata) map[string]string {
 	tags := map[string]string{
 		"title":                 meta.Title,
@@ -107,6 +138,8 @@ func toTags(meta Metadata) map[string]string {
 		"isrc":                  meta.ISRC,
 		"replaygain_track_gain": meta.ReplaygainTrackGain,
 		"replaygain_album_gain": meta.ReplaygainAlbumGain,
+		"replaygain_track_peak": meta.ReplaygainTrackPeak,
+		"replaygain_album_peak": meta.ReplaygainAlbumPeak,
 		"source_platform":       strings.ToUpper(strings.TrimSpace(meta.SourcePlatform)),
 		"source_track_id":       meta.SourceTrackID,
 		"source_album_id":       meta.SourceAlbumID,

internal/audio/tag/tagger_test.go

@@ -25,13 +25,17 @@ func TestToTagsTrackDiscFormatting(t *testing.T) {
 
 func TestToTagsTotalsAndSourceFields(t *testing.T) {
 	tags := toTags(Metadata{
-		TrackNumber:    3,
-		TrackTotal:     12,
-		DiscNumber:     1,
-		DiscTotal:      2,
-		ISRC:           "USABC1234567",
-		SourcePlatform: "qobuz",
-		SourceTrackID:  "t1",
+		TrackNumber:         3,
+		TrackTotal:          12,
+		DiscNumber:          1,
+		DiscTotal:           2,
+		ISRC:                "USABC1234567",
+		ReplaygainTrackGain: "-7.25 dB",
+		ReplaygainAlbumGain: "-8.1 dB",
+		ReplaygainTrackPeak: "0.989",
+		ReplaygainAlbumPeak: "1.001",
+		SourcePlatform:      "qobuz",
+		SourceTrackID:       "t1",
 	})
 	if tags["track"] != "03/12" {
 		t.Fatalf("track tag = %q", tags["track"])
@@ -48,6 +52,12 @@ func TestToTagsTotalsAndSourceFields(t *testing.T) {
 	if tags["source_platform"] != "QOBUZ" || tags["source_track_id"] != "t1" {
 		t.Fatalf("source tags missing: %+v", tags)
 	}
+	if tags["replaygain_track_gain"] != "-7.25 dB" || tags["replaygain_album_gain"] != "-8.1 dB" {
+		t.Fatalf("replaygain gain tags missing: %+v", tags)
+	}
+	if tags["replaygain_track_peak"] != "0.989" || tags["replaygain_album_peak"] != "1.001" {
+		t.Fatalf("replaygain peak tags missing: %+v", tags)
+	}
 }
 
 func TestBuildFFmpegArgsWithCover(t *testing.T) {
@@ -56,7 +66,7 @@ func TestBuildFFmpegArgsWithCover(t *testing.T) {
 	if err := os.WriteFile(cover, []byte("x"), 0o644); err != nil {
 		t.Fatalf("write cover: %v", err)
 	}
-	args := buildFFmpegArgs("in.flac", "out.flac", Metadata{Title: "x"}, cover)
+	args := buildFFmpegArgs("in.flac", "out.flac", Metadata{Title: "x"}, cover, "flac")
 	foundInput2 := false
 	foundAttach := false
 	for i := 0; i < len(args)-1; i++ {
@@ -71,3 +81,26 @@ func TestBuildFFmpegArgsWithCover(t *testing.T) {
 		t.Fatalf("missing cover args: %v", args)
 	}
 }
+
+func TestBuildFFmpegArgsSkipsCoverForUnsupportedContainer(t *testing.T) {
+	tmp := t.TempDir()
+	cover := filepath.Join(tmp, "cover.jpg")
+	if err := os.WriteFile(cover, []byte("x"), 0o644); err != nil {
+		t.Fatalf("write cover: %v", err)
+	}
+	args := buildFFmpegArgs("in.opus", "out.opus", Metadata{Title: "x"}, cover, "opus")
+	for i := 0; i < len(args)-1; i++ {
+		if args[i] == "-i" && args[i+1] == cover {
+			t.Fatalf("unexpected cover input for opus: %v", args)
+		}
+	}
+}
+
+func TestTaggedTempPathPreservesExtension(t *testing.T) {
+	if got := taggedTempPath("/tmp/song.flac"); got != "/tmp/song.tmp.flac" {
+		t.Fatalf("taggedTempPath(flac)=%q", got)
+	}
+	if got := taggedTempPath("/tmp/song"); got != "/tmp/song.tmp" {
+		t.Fatalf("taggedTempPath(noext)=%q", got)
+	}
+}