Add logging for server subscriptions in ServerMessageService

backend/src/main/java/com/alttd/altitudeweb/services/chat/to_server/ServerMessageService.java

@@ -27,6 +27,7 @@ public class ServerMessageService {
 
     public synchronized SseEmitter subscribe(String server) {
         servers.add(server);
+        log.info("Server {} subscribed", server);
         SseEmitter emitter = eventPublisher.subscribe(server, this::handleSessionEnd);
         if (latestPlayerState != null) {
             eventPublisher.sendToUser(server, PLAYER_STATE_CHANNEL, latestPlayerState);