fix(17-02): fix cover art replace and remove

Three issues fixed:

1. asBytes() helper for []interface{} → []byte conversion — same
   float64 deserialization issue as numeric fields. Cover art data
   from the frontend arrives as []interface{} of float64, not []byte.

2. DB sync for cover art — was a placeholder no-op. Now saves image
   to covers cache dir (content-hash dedup + thumbnail generation),
   upserts cover_art row, and updates release_groups.cover_art_id.
   Clear sets cover_art_id to NULL on linked release groups.

3. Frontend ReadFile returns base64 string (Go []byte JSON encoding),
   not number[]. Decode with atob() before creating Uint8Array for
   preview blob URL.

backend/tagwriter/mp3.go

@@ -96,7 +96,7 @@ func applyCoverArtChanges(tag *id3v2.Tag, changes TagChanges) {
 
 	apicID := tag.CommonID("Attached picture")
 
-	data, isBytes := val.([]byte)
+	data, isBytes := asBytes(val)
 	if isBytes && len(data) > 0 {
 		tag.DeleteFrames(apicID)
 		tag.AddAttachedPicture(id3v2.PictureFrame{