fix(engine): remove -shortest from muxVideoWithAudio (#1648) (#1650)

FFmpeg 6.0 (bundled by ffmpeg-static) has a regression where -shortest
combined with -c:v copy over-truncates the video stream while leaving
audio untouched. The flag is also redundant — the audio mixer already
pads/caps all tracks to totalDuration via apad=whole_dur and -t.

Closes #1648

packages/engine/src/services/chunkEncoder.ts

@@ -768,7 +768,7 @@ export async function muxVideoWithAudio(
     // output container metadata. `-c:v copy` is retained; no re-encode.
     args.push("-r", fpsToFfmpegArg(fps));
   }
-  args.push("-shortest", "-y", outputPath);
+  args.push("-y", outputPath);
 
   const processTimeout = config?.ffmpegProcessTimeout ?? DEFAULT_CONFIG.ffmpegProcessTimeout;
   const result = await runFfmpeg(args, { signal, timeout: processTimeout });