feat(voice): fork @discordjs/voice with video receive support

Fork @discordjs/voice 0.19.2 into vendor/discord-voice-fork and patch the
voice gateway handshake so Discord sends camera/screen-share RTP video:

- Identify payload now declares video:true + streams:[] (derived from
  Discord-RE/Discord-video-stream) - this is what makes Discord deliver
  H264 (PT 96-127) to the voice socket. Previously the client never
  declared video capability, so Discord omitted all video SSRCs/packets.
- op-12 Speaking handler maps streams[].ssrc -> videoSSRC alongside the
  audio SSRC, so ssrcMap carries camera/screen-share stream IDs.
- SSRCMap.get() now also resolves video SSRCs (video RTP arrives on a
  different SSRC than audio), enabling attribution for videoReceiver.ts.
- Both dist/index.js (CJS) and dist/index.mjs (ESM) patched; 3 isolated
  hunks vs upstream, verified by diff.
- package.json points @discordjs/voice -> file:vendor/discord-voice-fork
  (pnpm lockfile updated, CI --frozen-lockfile compatible).
- .gitignore: replace global dist/ with per-service explicit patterns so
  the vendored fork's dist/ is committed while build outputs stay ignored.
- tests: +3 SSRCMap videoSSRC cases (190 total, all pass).

The receive-side pipeline (H264Depacketizer -> .h264 -> muxToMp4 -> MP4)
already exists in videoReceiver.ts; this unblocks it by making Discord
actually deliver video packets.

.gitignore

@@ -1,7 +1,9 @@
 node_modules
 /recordings
 .env
-dist/
+services/discord-gateway/dist/
+services/backend/dist/
+services/frontend/frontend/dist/
 public/app/
 .muxer-queue.**
 .claude/
@@ -10,7 +12,6 @@ logs/
 .codegraph/
 worktrees/
 .worktrees/
-services/frontend/frontend/dist/
 target/
 nix/
 # Gitea CI runner logs