refactor(gateway): remove screen-share / GoLive feature entirely

Drop the Discord Go Live (screen share) stack across the discord-gateway:
- delete src/goLive/ (19 modules: Streamer, Demuxer, encoders, WebRTC wrapper, native loader, etc.)
- delete native/libdatachannel-min/ N-API binding + flake native build + LD_LIBRARY_PATH wiring
- delete screenShareController.ts and screen-share tests (goLive-port, golive-*, demuxerNut, screenShareInput)
- mediaSource.ts: remove Invidious helpers + downloadScreenInput (YouTube full-file download)
- mediaTypes.ts: drop ScreenShare* types, narrow MediaMode to 'music' and DiscordPlayerOwner to non-screen
- media.handler.ts: remove screen branch, screenController/screenPlayback, voice-disconnect/reconnect accessor
- commandHandler.ts: stop passing getVoiceStatus / setVoiceController into MediaHandler
- media handler now only handles music; music queue/playback/status untouched

Verification: tsc --noEmit clean, biome clean on touched files, no lingering goLive/screenShare refs in BE/FE/gateway.

services/discord-gateway/tests/golive-demux-e2e.ts

@@ -1,40 +0,0 @@
-// Phase 2 E2E: Demuxer on a real ffmpeg-generated H264 file.
-// Run: npx tsx tests/golive-demux-e2e.ts
-
-import { createReadStream } from "node:fs";
-import { demux } from "../src/goLive/Demuxer.js";
-
-const input = process.argv[2] ?? "/tmp/sample.h264";
-const { video, close } = await demux(createReadStream(input), {
-  format: "h264",
-});
-
-console.log(
-  "video:",
-  JSON.stringify({
-    codecName: video.codecName,
-    width: video.width,
-    height: video.height,
-    duration: video.duration,
-    fps: Math.round(video.framerate_num / video.framerate_den),
-  }),
-);
-
-let count = 0;
-let keyframes = 0;
-let bytes = 0;
-video.stream.on("data", (frame: { data: Buffer; keyframe: boolean }) => {
-  count++;
-  bytes += frame.data.length;
-  if (frame.keyframe) keyframes++;
-});
-video.stream.on("end", () => {
-  console.log(`frames: ${count} (${keyframes} keyframes), ${bytes} bytes`);
-  close();
-  process.exit(0);
-});
-video.stream.on("error", (e: unknown) => {
-  console.error("stream error:", e);
-  close();
-  process.exit(1);
-});