Calendar, Contacts (CalDAV, CardDAV) integration

[markussbk]

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New feature idea

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Consider implementing a CalDAV, CardDAV server and possibly a nice calendar WebUI :)
There exists:

• Radicale - https://radicale.org/v3.html
• Baikal - https://sabre.io/baikal/ (comes with administration WebUI to create users and calendars, but no way to check out saved calendar events)
• sabre/dav - https://sabre.io/dav/ (php CalDAV+CardDAV framework)

From these options radicale would be ok, check out how Opencloud implements this currently (no WebUI currently, Opencloud is a proxy to Radicale, it autoprovisions calendars for each user)

• https://docs.opencloud.eu/docs/admin/configuration/radicale-integration/
• https://github.com/opencloud-eu/opencloud-compose/blob/main/radicale/radicale.yml

Ideally there should also be a WebUI, where I can login and check out what calendars I have (eg. Nextcloud)

Validations

• Check the feature is not already implemented in the project.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.
• Check that the feature is technically feasible and aligns with the project's goals.