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>

.env.example

@@ -43,6 +43,10 @@ MEET_API_URL=https://meet.ladill.com/api/service/v1
 MEET_API_KEY_EVENTS=
 MEET_WEBHOOK_SECRET_EVENTS=
 
+# Ladill Mail calendar (sync events to the webmail utility calendar)
+LADILL_MAIL_API_URL=https://mail.ladill.com/api/service/v1
+MAIL_API_KEY_EVENTS=
+
 # Inbound service API (Meet calls Events)
 EVENTS_API_KEY_MEET=