Previous code only added ffmpeg -re when input was a string URL. Screen
share passes a Readable pipe (yt-dlp merge -> stdout) delivered at network
speed (bursts + stalls). Without -re the encoder slurps it instantly and,
when the merge stalls, x264 -r 30 force-duplicates the last held frame
~30x -> viewer sees ~1fps while WebRTC still paces 30fps. Add -re for all
inputs so the encoder paces at the stream's native PTS rate and emits a
fresh picture every frame.