Sync Ladill Events to the Mail utility calendar.

Events upsert calendar entries by stable external_ref keys, prefer the
owner's linked mailbox, and Meet skips mail sync for events-sourced rooms
so linked virtual sessions appear only once on the calendar.

Co-authored-by: Cursor <cursoragent@cursor.com>

config/mail_calendar.php

@@ -0,0 +1,6 @@
+<?php
+
+return [
+    'api_url' => env('LADILL_MAIL_API_URL', 'https://mail.ladill.com/api/service/v1'),
+    'api_key' => env('MAIL_API_KEY_EVENTS'),
+];