Push sync for calendars, tasks, and contacts (CalDAV and CardDAV)

Is your feature request related to a problem? Please describe.
Nextcloud is clearly maturing fast. Thanks to everyone for the hard work. To bring it to parity with other first class cloud services, it would be awesome if Nextcloud could provide push notifications for CalDAV and CardDAV. Changes to calendars and tasks would benefit from faster propagation without needing to resort to rapid fetch intervals, which are resource intensive. It's frustrating when changes across devices are not in sync and create version control issues when editing on multiple devices.

It appears to be possible to enable push for CalDAV and CardDAV on Apple iOS, iPadOS, and macOS devices. Not sure about other devices.

Describe the solution you'd like
Implement something like this from FastMail. Unclear what the technical backend is. Perhaps they would be willing to share. For iOS, iPadOS, and macOS, it appears this involved collaboration with Apple, although not sure.
https://fastmail.blog/2016/10/10/push-sync-for-contacts-calendars-now-available-on-ios/

Describe alternatives you've considered
Faster fetch intervals are imperfect, drain resources (especially for battery powered devices. Z-push is an option but clunky and also resource intensive. It is inefficient, since it just converts CalDAV and CardDAV to Microsoft ActiveSync. This also creates reliance on a new protocol.

Thanks again, and completely understand that this is "nice to have" not "need to have." But, given how Nextcloud is maturing, it would be awesome to bring it to parity with cloud services others offer.

The issue names no files, tests, or entry points. Start by determining how CalDAV and CardDAV currently synchronize and what technical backend could provide push notifications, then define supported clients, protocol boundaries, and measurable completion criteria.