Create backend for calendar automation

[friedger]

The backend should have the following features:

• when a new booking request is created, a calendar entry for the room is created with status "tentative". The calendar entry id is stored in the database.
• when a booking is confirmed, the status is changed to "confirmed"

The details of the booking request should be kept private, just the name is public.

[friedger]

A first start is sending an email to CHB on a new request.

@MathieuHP started work at https://github.com/CommonsHub/bookingsystem-dbtrigger

[friedger]

Next calendar entry, nostr message, etc.. all in dbtrigger.