feat(voice): add voice transmit functionality for real-time audio streaming

- Add VoiceTransmitter class to handle PCM audio from backend/browser to Discord
- Implement voice:transmit:start and voice:transmit:stop command handlers
- Upsample 24kHz mono PCM to 48kHz stereo for Discord compatibility
- Encode PCM to Opus and stream via browser-bridge player owner
- Subscribe to Redis channel backend:voice:transmit for real-time PCM data

Features:
- Backend can send PCM audio (24kHz mono s16le base64) via Redis
- Automatic upsampling and encoding to Discord-compatible format
- Clean start/stop lifecycle with resource cleanup

This completes the bidirectional voice streaming:
- Listen: Discord → Backend (already working via voicePcmData broadcast)
- Transmit: Backend → Discord (now implemented)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/discord-gateway/src/modules/command-handler/commandHandler.ts

@@ -3,6 +3,7 @@ import type { Client } from "discord.js-selfbot-v13";
 import Redis from "ioredis";
 import { config } from "../../shared/config/config.js";
 import { discordPlayer } from "../voice-recording/player.js";
+import { voiceTransmitter } from "../voice-recording/transmitter.js";
 import type { VoiceController } from "../voice-recording/voiceController.js";
 
 const logger = createChildLogger("command-handler");
@@ -130,6 +131,12 @@ export class CommandHandler {
         case "voice:channels":
           reply = await this.handleVoiceChannels(cmd);
           break;
+        case "voice:transmit:start":
+          reply = await this.handleVoiceTransmitStart(cmd);
+          break;
+        case "voice:transmit:stop":
+          reply = await this.handleVoiceTransmitStop(cmd);
+          break;
         case "guilds:list":
           reply = await this.handleListGuilds(cmd);
           break;
@@ -370,6 +377,63 @@ export class CommandHandler {
     };
   }
 
+  private async handleVoiceTransmitStart(cmd: BackendCommand): Promise<CommandReply> {
+    if (!discordPlayer.isConnected()) {
+      return {
+        id: cmd.id,
+        success: false,
+        data: null,
+        error: "Not connected to voice channel",
+      };
+    }
+
+    try {
+      // Create a new Redis connection for the transmitter
+      const transmitRedis = new Redis(config.REDIS_URL);
+      await voiceTransmitter.start(transmitRedis);
+
+      const status = voiceTransmitter.getStatus();
+      logger.info({ status }, "Voice transmit started");
+
+      return {
+        id: cmd.id,
+        success: true,
+        data: status,
+      };
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err);
+      logger.error({ error: message }, "Failed to start voice transmit");
+      return {
+        id: cmd.id,
+        success: false,
+        data: null,
+        error: message,
+      };
+    }
+  }
+
+  private async handleVoiceTransmitStop(cmd: BackendCommand): Promise<CommandReply> {
+    try {
+      await voiceTransmitter.stop();
+      logger.info("Voice transmit stopped");
+
+      return {
+        id: cmd.id,
+        success: true,
+        data: { status: "stopped" },
+      };
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err);
+      logger.error({ error: message }, "Failed to stop voice transmit");
+      return {
+        id: cmd.id,
+        success: false,
+        data: null,
+        error: message,
+      };
+    }
+  }
+
   // ---- Status publishing ----
 
   private publishVoiceStatus(): void {