Phase 2 backend: LiveKit voice integration

Backend:
- internal/voice/client.go: LiveKit token generation, room management, participant listing
- internal/voice/handlers.go: POST /voice/token, GET /voice/rooms/{roomID}/participants
- internal/gateway/events.go: added VOICE_JOIN, VOICE_LEAVE, VOICE_MUTE, VOICE_DEAFEN events
- cmd/server/main.go: wired voice client and routes under /api/v1/voice
- Docker: livekit.yaml config verified

Frontend deps:
- Added livekit-client and @livekit/components-react packages

internal/gateway/events.go

@@ -14,6 +14,10 @@ const (
 	EventMemberRemove   = "SERVER_MEMBER_REMOVE"
 	EventPresenceUpdate = "PRESENCE_UPDATE"
 	EventTypingStart    = "TYPING_START"
+	EventVoiceJoin      = "VOICE_JOIN"
+	EventVoiceLeave     = "VOICE_LEAVE"
+	EventVoiceMute      = "VOICE_MUTE"
+	EventVoiceDeafen    = "VOICE_DEAFEN"
 )
 
 // Event represents a WebSocket event sent to clients.