fix(voice): add HTTP API fallback for transmit commands

- Add POST /api/voice/command endpoint in backend
- Frontend transmit now sends commands via dedicated WebSocket connection
  with HTTP API fallback if WebSocket fails
- Fixes issue where transmit start command was never received by discord-gateway

Previously commands were sent via the existing dashboard WebSocket,
which may not be connected when the Transmit button is pressed.
Now each command opens a fresh WebSocket connection with HTTP fallback.

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

services/backend/src/modules/voice/voice.routes.ts

@@ -5,6 +5,7 @@ import {
   handleDisconnectVoice,
   handleGetVoiceChannels,
   handleGetVoiceStatus,
+  handleVoiceCommand,
 } from "./voice.controller.js";
 
 export function createVoiceRouter(): Router {
@@ -22,5 +23,8 @@ export function createVoiceRouter(): Router {
   // GET /api/guilds/:guildId/voice-channels
   router.get("/guilds/:guildId/voice-channels", handleGetVoiceChannels);
 
+  // POST /api/command — send arbitrary voice command (transmit start/stop)
+  router.post("/command", handleVoiceCommand);
+
   return router;
 }