fix: data race in PushChatHistory WebSocket writes

backend/chat/src/ws/ws.go

@@ -105,9 +105,11 @@ func (h *Hub) PushChatHistory(conn *websocket.Conn, room string, limit int) {
 		opp := len(messages) - 1 - i
 		messages[i], messages[opp] = messages[opp], messages[i]
 	}
+	h.mu.Lock()
 	for _, msg := range messages {
 		conn.WriteJSON(msg)
 	}
+	h.mu.Unlock()
 }
 
 // HandleConnection handles a new WebSocket connection.