feat(library): ISRC duplicate detection beyond FLAC with review sheet

The download-time ISRC index now also parses mp3, m4a, ogg, and opus
tags through the existing native readers instead of skipping everything
but .flac. Library settings gain a Review duplicates sheet that groups
history and local-library rows sharing an ISRC via SQL over the
attached databases (no filesystem walk, SAF-safe), shows quality
badges, and offers keep-best and per-copy delete with confirmation.

go_backend/cue_duplicate_supplement_test.go

@@ -159,6 +159,40 @@ func writeTestFlacWithISRC(t *testing.T, path, isrc string) {
 	}
 }
 
+func TestISRCIndexCoversNonFlacFormats(t *testing.T) {
+	dir := t.TempDir()
+	flacPath := filepath.Join(dir, "a.flac")
+	mp3Path := filepath.Join(dir, "b.mp3")
+	writeTestFlacWithISRC(t, flacPath, "USAA00000011")
+	mp3Data := buildID3v23Tag(
+		id3TextFrame("TIT2", "Song"),
+		id3TextFrame("TSRC", "usbb00000022"),
+	)
+	if err := os.WriteFile(mp3Path, mp3Data, 0600); err != nil {
+		t.Fatal(err)
+	}
+	// Unsupported/untagged formats must stay invisible to the index.
+	if err := os.WriteFile(filepath.Join(dir, "c.wav"), []byte("RIFF"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	defer InvalidateISRCCache(dir)
+
+	if got := readFileISRC(mp3Path); got != "usbb00000022" {
+		t.Fatalf("readFileISRC mp3 = %q", got)
+	}
+	if got := readFileISRC(filepath.Join(dir, "c.wav")); got != "" {
+		t.Fatalf("readFileISRC wav = %q", got)
+	}
+
+	idx := buildISRCIndex(dir)
+	if path, ok := idx.lookup("USAA00000011"); !ok || path != flacPath {
+		t.Fatalf("flac lookup = %q/%v", path, ok)
+	}
+	if path, ok := idx.lookup("USBB00000022"); !ok || path != mp3Path {
+		t.Fatalf("mp3 lookup = %q/%v", path, ok)
+	}
+}
+
 func TestISRCIndexIncrementalRebuild(t *testing.T) {
 	dir := t.TempDir()
 	trackA := filepath.Join(dir, "a.flac")

go_backend/duplicate.go

@@ -108,7 +108,7 @@ func buildISRCIndex(outputDir string) *ISRCIndex {
 		}
 
 		ext := strings.ToLower(filepath.Ext(path))
-		if ext != ".flac" {
+		if !isrcIndexExts[ext] {
 			return nil
 		}
 
@@ -141,7 +141,7 @@ func buildISRCIndex(outputDir string) *ISRCIndex {
 			go func() {
 				defer wg.Done()
 				for i := range tasks {
-					isrcs[i] = strings.ToUpper(readFlacISRC(toParse[i].path))
+					isrcs[i] = strings.ToUpper(readFileISRC(toParse[i].path))
 				}
 			}()
 		}
@@ -170,6 +170,38 @@ func buildISRCIndex(outputDir string) *ISRCIndex {
 	return idx
 }
 
+// isrcIndexExts are the formats the download pipeline can produce; each has
+// a native tag reader that stops at the metadata blocks.
+var isrcIndexExts = map[string]bool{
+	".flac": true,
+	".mp3":  true,
+	".m4a":  true,
+	".ogg":  true,
+	".opus": true,
+}
+
+// readFileISRC reads the ISRC tag using the native reader for the format.
+// Returns "" for unsupported formats, unreadable files, or missing tags.
+func readFileISRC(path string) string {
+	switch strings.ToLower(filepath.Ext(path)) {
+	case ".flac":
+		return readFlacISRC(path)
+	case ".mp3":
+		if meta, err := ReadID3Tags(path); err == nil && meta != nil {
+			return strings.TrimSpace(meta.ISRC)
+		}
+	case ".m4a":
+		if meta, err := ReadM4ATags(path); err == nil && meta != nil {
+			return strings.TrimSpace(meta.ISRC)
+		}
+	case ".ogg", ".opus":
+		if meta, err := ReadOggVorbisComments(path); err == nil && meta != nil {
+			return strings.TrimSpace(meta.ISRC)
+		}
+	}
+	return ""
+}
+
 // readFlacISRC extracts the ISRC Vorbis comment from a FLAC file by walking
 // the metadata block headers and reading only the VORBIS_COMMENT payload;
 // picture and padding blocks are seeked past, never loaded. Returns "" when