adding caldav/carddav next to dovecot

[Plutocrat]

I think rather than Sabre's DAV, you're after their Baikal server.

     https://sabre.io/baikal/

Sabre is also the guts behind Nextcloud's Contacts and Calendar dav sync, which you might want to look into, especially if you can see a need for sharing contacts among a group of people, as well as files, group chat and all the other features of Nextcloud. To bring it back onto topic, Nextcloud can also be used as a webmail interface to dovecot, of course.

     https://nextcloud.com/

P.