Here's how to ring the doorbell from a terminal:
mosquitto_pub -h 192.168.0.101 -m '{"action":"single","battery":100,"last_seen":1664390305071,"linkquality":0}' -t "sensors/doorbell"
The doorbell subscribes to DOORBELL_MQTT_TOPIC, defaulting to sensors/Doorbell, and now reports the configured topic on startup.