Webhooks and events

Register signed HTTPS endpoints for operation.completed, charging_session.status_changed, bill.finalized, display.delivery_changed and the charger events (station.connected, station.disconnected, station.booted, connector.status_changed, station.fault_opened, station.fault_cleared, meter.sampled; optional stations filter and meter sample interval), inspect delivery attempts and poll the same events by cursor with type and station_id filters. Requires webhooks:write to manage endpoints plus the read scope of each subscribed event. Deliveries are at-least-once with backoff; deduplicate by event id.