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/goLive/Demuxer.ts

@@ -161,6 +161,11 @@ export async function demux(
     // stderr and are parsed for dimensions/fps.
     "-loglevel",
     "info",
+    // Input format hint: prepareStream always emits raw AnnexB H264 on
+    // pipe:0. Raw H264 has NO magic header, so ffmpeg's auto-detection
+    // fails with "Invalid data found when processing input" whenever the
+    // first bytes arrive late/buffered. Pin the demuxer input format.
+    ...(isStream ? ["-f", "h264"] : []),
     "-i",
     isStream ? "pipe:0" : input,
     "-c:v",