Add ntfy.sh push notifications for booking events

Sends instant push notification on new booking, modification, and
cancellation. Set ntfy_topic in application/config/google_calendar.php
to activate. No-ops silently if topic is empty.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

application/config/google_calendar.php

@@ -3,3 +3,4 @@ defined('BASEPATH') OR exit('No direct script access allowed');
 
 $config['gcal_service_account_path'] = APPPATH . 'config/service-account-key.json';
 $config['gcal_evelin_calendar_id']   = 'katozoltan0804@gmail.com'; // Evelin's Calendar ID (e.g. her Gmail address)
+$config['ntfy_topic']                = ''; // ntfy.sh topic for push notifications (e.g. 'studiobeve-bookings-x7k2')