fix(gateway): revert CO2 range check and add MQTT auto-reconnect

- Revert CO2 range check — filtering handled by database team
- Exit on MQTT disconnection so systemd restarts automatically
- Add on_disconnect callback with SystemExit(1)

gateway/gateway.py

@@ -79,11 +79,21 @@ class Gateway:
             else:
                 log.error(f"Failed to connect to MQTT broker — reason code: {reason_code}")
 
+        # Callback triggered on disconnection — exit so systemd restarts the gateway.
+        # This handles cases where the broker becomes unreachable after the initial
+        # connection (e.g. network instability). Systemd will restart the gateway
+        # after RestartSec=10 seconds, retrying the connection automatically.
+        def on_disconnect(client, userdata, flags, reason_code, properties):
+            log.error(f"Disconnected from MQTT broker — reason code: {reason_code}")
+            log.error("Exiting — systemd will restart the gateway")
+            raise SystemExit(1)
+
         # Callback to confirm message delivery to broker
         def on_publish(client, userdata, mid, reason_code, properties):
             log.info(f"Message confirmed by broker — mid: {mid}")
 
         self.mqttc.on_connect = on_connect
+        self.mqttc.on_disconnect = on_disconnect
         self.mqttc.on_publish = on_publish
 
         self.mqttc.connect(self.mqtt_broker, self.mqtt_port)