Fix FFmpeg WebM VP9 Alpha decoding issue during transcode

server.mjs

@@ -387,7 +387,12 @@ async function runSequentialEncodes(session, inputVideoPath, inputAudioPath, tar
     session.currentStatus = `Rendere ${i+1}/${targetFormats.length}: ${formatId.toUpperCase()}...`;
     saveHistory();
     
-    const ffmpegArgs = ['-y', '-i', inputVideoPath];
+    const ffmpegArgs = ['-y'];
+    if (inputVideoPath.toLowerCase().endsWith('.webm')) {
+      ffmpegArgs.push('-c:v', 'libvpx-vp9');
+    }
+    ffmpegArgs.push('-i', inputVideoPath);
+    
     if (inputAudioPath) {
       ffmpegArgs.push('-i', inputAudioPath);
     }