M5-T08: add MQTT, HA Discovery, and Modbus polling config fields

.env.example

@@ -28,3 +28,20 @@ TICKTICK_CLIENT_ID=
 TICKTICK_CLIENT_SECRET=
 TICKTICK_TOKEN=
 HOME_ASSISTANT_ACTION_TASK_PROJECT_ID=
+
+# Optional: Modbus polling (global kill-switch; default true).
+# MODBUS_POLLING_ENABLED=true
+
+# Optional: MQTT broker connection.
+# Leave MQTT_ENABLED=false (or unset) when MQTT is not needed.
+MQTT_ENABLED=false
+MQTT_BROKER_HOST=
+MQTT_BROKER_PORT=1883
+MQTT_USERNAME=
+MQTT_PASSWORD=
+MQTT_TLS_ENABLED=false
+
+# Optional: Home Assistant MQTT Discovery.
+# Requires MQTT_ENABLED=true and a running MQTT broker.
+HA_DISCOVERY_ENABLED=false
+HA_DISCOVERY_PREFIX=homeassistant