Fix MP4 mux for web playback: add faststart and genpts

- Add -movflags +faststart to move moov atom to start
- Add -fflags +genpts for proper timestamp generation
- Fixes jerky playback and video/audio desync

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tasks/streaming.py

@@ -411,6 +411,8 @@ def run_stream(
             "ffmpeg", "-y",
             "-i", str(hls_playlist),
             "-c", "copy",  # Just copy streams, no re-encoding
+            "-movflags", "+faststart",  # Move moov atom to start for web playback
+            "-fflags", "+genpts",  # Generate proper timestamps
             str(final_mp4)
         ]
         logger.info(f"Muxing HLS to MP4: {' '.join(mux_cmd)}")