try to fix websocket shutdown

backend/src/utilities/packet_broadcaster.py

@@ -6,6 +6,8 @@ from typing import Any, Dict, List, Optional
 
 logger = logging.getLogger("packet_broadcaster")
 
+_SHUTDOWN_SENTINEL: Dict[str, Any] = {"type": "__broadcaster_shutdown__"}
+
 
 class PacketBroadcaster:
     """Manage subscriber queues and publish packet events."""
@@ -96,5 +98,6 @@ class PacketBroadcaster:
             try:
                 while not queue.empty():
                     queue.get_nowait()
+                queue.put_nowait(_SHUTDOWN_SENTINEL)
             except Exception:
                 pass