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/src/modules/command-handler/commandHandler.ts

@@ -83,12 +83,7 @@ export class CommandHandler {
 
     // Create domain-specific handlers with their dependencies
     this.voiceHandler = new VoiceHandler(client, voiceController);
-    this.mediaHandler = new MediaHandler(client, () =>
-      voiceController.getStatus(),
-    );
-    // Give media handler access to disconnect/reconnect voice around screen
-    // share (GoLive needs its own WebRTC connection).
-    this.mediaHandler.setVoiceController(() => voiceController);
+    this.mediaHandler = new MediaHandler();
     this.guildHandler = new GuildHandler(client);
     this.moderationHandler = new ModerationHandler(client);