fix(server): better embedded artwork extraction with ffmpeg (#3860)

- `-map 0:v` selects all video streams from the input
- `-map -0:V` excludes all "main" video streams (capital V)

This combination effectively selects only the attached pictures

Signed-off-by: Deluan <deluan@navidrome.org>

core/ffmpeg/ffmpeg.go

@@ -29,7 +29,7 @@ func New() FFmpeg {
 }
 
 const (
-	extractImageCmd = "ffmpeg -i %s -an -vcodec copy -f image2pipe -"
+	extractImageCmd = "ffmpeg -i %s -map 0:v -map -0:V -vcodec copy -f image2pipe -"
 	probeCmd        = "ffmpeg %s -f ffmetadata"
 )