fix: Remove await from synchronous Modbus client disconnect call

The Modbus client disconnect method is synchronous, not async

src/dashboard/api.py

@@ -798,7 +798,7 @@ async def get_signals():
                     # Add OPC UA fallback signals
                     signals.extend(_create_fallback_signals(station_id, pump_id))
             
-            await modbus_client.disconnect()
+            modbus_client.disconnect()
         else:
             logger.warning("modbus_connection_failed_using_fallback")
             # Fallback to mock data if Modbus connection fails