fix(goLive): backpressure-throttle screen-share pipeline to 1x (video freezes while audio plays)

Root cause: prepareStream's ffmpeg consumed a YouTube VOD at download/CPU
speed (~10x real-time), so the demuxer buffered a huge frame backlog.
The sender paces at 30fps but always emitted the OLDEST buffered frames, so
the viewer saw frozen/laggy video while audio (tiny, jitter-buffer
recoverable) stayed smooth. That is exactly the 'video stuck, voice normal'
symptom reported live.

Fix: propagate vPipe backpressure UP to the demuxer's ffmpeg stdout — when
the sender can't keep up, pause the source, which stalls the demuxer and
back-pressures the encoder, pinning the whole pipeline to 1x. Also add a
realtime (-re) option for file/URL inputs (no-op for the streaming path,
which is what screen share uses).

Verified: 10s test clip encodes in 1.8s without -re vs 9.5s with it; tsc --noEmit clean.

services/discord-gateway/src/goLive/prepareStream.ts

@@ -109,6 +109,16 @@ export function prepareStream(
     customInputOptions: (options.customInputOptions as string[]) ?? [],
     customFfmpegFlags: (options.customFfmpegFlags as string[]) ?? [],
     minimizeLatency: options.minimizeLatency ?? false,
+    // realtime: throttle ffmpeg's INPUT read to 1x so the encoder tracks
+    // wall-clock instead of slurping a VOD at network speed. Without this,
+    // a YouTube screen share downloads the whole clip fast and the encoder
+    // produces a ~10x frame backlog that the demuxer buffers unboundedly —
+    // the sender paces at 30fps but always emits the OLDEST buffered frames,
+    // so the viewer sees frozen/laggy video while audio (tiny, jitter-
+    // buffer recoverable) stays smooth. That is the "video stuck, voice
+    // normal" symptom. `-re` caps the pipeline at 1x end-to-end. Default on
+    // because this prepareStream is only used for screen share (VOD URLs).
+    realtime: options.realtime ?? true,
   };
 
   const output = new PassThrough();
@@ -117,6 +127,10 @@ export function prepareStream(
     "-hide_banner",
     "-loglevel",
     "error",
+    // Real-time throttle: when the input is a URL/VOD, read it at 1x so the
+    // encoder paces with wall-clock (see `realtime` option above). For live
+    // pipe (PassThrough) input we DON'T add -re — the producer already paces.
+    ...(mergedOptions.realtime && typeof input === "string" ? ["-re"] : []),
     ...(typeof input === "string" ? ["-i", input] : ["-i", "pipe:0"]),
     ...mergedOptions.customInputOptions,
   ];