feat: populate M4A metadata in ReadFileMetadata and library scan

ReadFileMetadata now fills all tag fields (title, artist, album, ISRC,
lyrics, genre, label, copyright, composer, comment, track/disc number)
for M4A files using the new ReadM4ATags helper, matching the existing
behavior for FLAC, MP3, and Ogg.

scanM4AFile reads tags via ReadM4ATags instead of falling back to the
filename, and applies applyDefaultLibraryMetadata for missing fields
(consistent with FLAC/MP3 scan path).

Remove the '&& ext != ".m4a"' guard in cover cache so M4A cover art
is extracted and cached during library scans.

go_backend/exports.go

@@ -739,6 +739,26 @@ func ReadFileMetadata(filePath string) (string, error) {
 			}
 		}
 	} else if isM4A {
+		meta, err := ReadM4ATags(filePath)
+		if err == nil && meta != nil {
+			result["title"] = meta.Title
+			result["artist"] = meta.Artist
+			result["album"] = meta.Album
+			result["album_artist"] = meta.AlbumArtist
+			result["date"] = meta.Date
+			if meta.Date == "" {
+				result["date"] = meta.Year
+			}
+			result["track_number"] = meta.TrackNumber
+			result["disc_number"] = meta.DiscNumber
+			result["isrc"] = meta.ISRC
+			result["lyrics"] = meta.Lyrics
+			result["genre"] = meta.Genre
+			result["label"] = meta.Label
+			result["copyright"] = meta.Copyright
+			result["composer"] = meta.Composer
+			result["comment"] = meta.Comment
+		}
 		quality, qualityErr := GetM4AQuality(filePath)
 		if qualityErr == nil {
 			result["bit_depth"] = quality.BitDepth
@@ -1960,8 +1980,15 @@ func ReEnrichFile(requestJSON string) (string, error) {
 		}
 	}()
 
-	// Fetch lyrics
+	// Preserve existing lyrics when online enrichment does not return a replacement.
 	var lyricsLRC string
+	existingLyrics, existingLyricsErr := ExtractLyrics(req.FilePath)
+	if existingLyricsErr == nil && strings.TrimSpace(existingLyrics) != "" {
+		lyricsLRC = existingLyrics
+		GoLog("[ReEnrich] Preserving existing embedded/sidecar lyrics\n")
+	}
+
+	// Fetch lyrics
 	if req.EmbedLyrics {
 		client := NewLyricsClient()
 		durationSec := float64(req.DurationMs) / 1000.0
@@ -2042,7 +2069,6 @@ func ReEnrichFile(requestJSON string) (string, error) {
 		return string(jsonBytes), nil
 	}
 
-	// MP3/Opus: return metadata map for Dart to use FFmpeg
 	// Don't cleanup cover temp — Dart needs it for FFmpeg embed
 	cleanupCover = false
 	result := map[string]interface{}{

go_backend/library_scan.go

@@ -293,7 +293,7 @@ func scanAudioFileWithKnownModTimeAndDisplayName(filePath, displayNameHint, scan
 	libraryCoverCacheMu.RLock()
 	coverCacheDir := libraryCoverCacheDir
 	libraryCoverCacheMu.RUnlock()
-	if coverCacheDir != "" && ext != ".m4a" {
+	if coverCacheDir != "" {
 		coverPath, err := SaveCoverToCacheWithHint(filePath, displayNameHint, coverCacheDir)
 		if err == nil && coverPath != "" {
 			result.CoverPath = coverPath
@@ -373,13 +373,30 @@ func scanFLACFile(filePath string, result *LibraryScanResult) (*LibraryScanResul
 }
 
 func scanM4AFile(filePath string, result *LibraryScanResult) (*LibraryScanResult, error) {
+	metadata, err := ReadM4ATags(filePath)
+	if err == nil && metadata != nil {
+		result.TrackName = metadata.Title
+		result.ArtistName = metadata.Artist
+		result.AlbumName = metadata.Album
+		result.AlbumArtist = metadata.AlbumArtist
+		result.ISRC = metadata.ISRC
+		result.TrackNumber = metadata.TrackNumber
+		result.DiscNumber = metadata.DiscNumber
+		result.ReleaseDate = metadata.Date
+		if result.ReleaseDate == "" {
+			result.ReleaseDate = metadata.Year
+		}
+		result.Genre = metadata.Genre
+	}
+
 	quality, err := GetM4AQuality(filePath)
 	if err == nil {
 		result.BitDepth = quality.BitDepth
 		result.SampleRate = quality.SampleRate
 	}
 
-	return scanFromFilename(filePath, "", result)
+	applyDefaultLibraryMetadata(filePath, "", result)
+	return result, nil
 }
 
 func scanMP3File(filePath string, result *LibraryScanResult) (*LibraryScanResult, error) {