chore(backend_server.py): set Client.last_seen on message receive

src/judas_server/backend/backend_server.py

@@ -145,6 +145,8 @@ class BackendServer:
                     self.logger.debug(f"[>] Sending ACK to {client}")
                     client.outbound += ack
 
+                    # set last seen
+                    client.last_seen = time.time()
                 else:
                     self._disconnect(client)