Fix server shut down on exit.

If there's a websocket being waited on the shut down won't actually
happen. Add a new event for shut down and send it to break the websocket
out of its loop.

game/server/eventstream/routes.py

@@ -14,6 +14,10 @@ async def event_stream(websocket: WebSocket) -> None:
     await websocket.accept()
     while True:
         if not (events := await EventStream.get()).empty:
+            if events.shutting_down:
+                await websocket.close()
+                return
+
             await websocket.send_json(
                 jsonable_encoder(
                     GameUpdateEventsJs.from_events(events, GameContext.get())