fix(goLive): black screen root cause — h264 muxer can't carry audio; disable self_video camera

ROOT CAUSE of empty GoLive tile (finally): prepareStream ran with
includeAudio: true + output -f h264. The h264 muxer cannot mux audio
('h264 muxer does not support any stream of type audio') → header write
fails -22 → stdout empty → Demuxer ffmpeg 'Invalid data found when
processing input' → 0 frames → black tile. Reproduced locally end-to-end
(13s backpressure delay + prepareStream + demux).

Fixes:
- screenShareController: includeAudio: false (video-only GoLive; demux
  path never delivers audio anyway)
- Demuxer: pin input format -f h264 for stream inputs (raw AnnexB H264
  has no magic header → auto-detect unreliable on delayed pipes)
- Streamer.signalStream: self_video: false — stop flipping on the bot's
  camera in Discord (user request; screen share ≠ camera)

Verified: local repro now emits 644 frames 1280x720 (was 0); tsc/biome/
vitest all green.

services/discord-gateway/src/modules/voice-recording/screenShareController.ts

@@ -105,7 +105,11 @@ export class ScreenShareController {
         frameRate: 30,
         bitrateVideo: 2500,
         bitrateVideoMax: 4000,
-        includeAudio: true,
+        // Video-only GoLive: the -f h264 output muxer cannot carry audio
+        // ("h264 muxer does not support any stream of type audio" → header
+        // write fails → empty stdout → demux 'Invalid data' → black tile).
+        // Audio is not delivered by the GoLive demux path anyway.
+        includeAudio: false,
         videoCodec: normalizeVideoCodec("H264"),
       });
       const { command } = prepared;