Federated free/busy availability exchange

Is your feature request related to a problem? Please describe.

When you invite people to a meeting, the free-busy information helps a lot with scheduling. If you invite people externally, this isn't available. But if we federate with another server, we have a trust relation.

Ideally, users on different Nextcloud instances would be able to see each other's free/busy information when scheduling meetings, without having to share their calendars or expose event details. This would make federated Nextcloud instances interoperable in the same way that Microsoft 365 can exchange availability between organizations.

Describe the solution you'd like

The ideal approach is to build on our federation mechanism, and using the existing free/busy mechanism in CalDAV.

If two instances are trusted by the admin to interoperate, we could allow free/busy information to (optionally) also be exchanged.

For large instances, you probably want to limit this to users who have each other (and their federated user ID) in their address book, providing an additional privacy/authorization layer.

We have to handle:

• federated identity check & federated server check (using OCM?)
• endpoint discovery for CalCAV
• privacy - same as with in-server free-busy

Describe alternatives you've considered

Doing it without federation is probably tricky. You'd want some kind of authorization mechanism, and I'm not sure how to do that. Would need, at least, some token-based approval mechanism I suppose.

Additional context

No response

The issue names no files, tests, or entry points. Begin by mapping the existing federation mechanism and CalDAV free/busy flow, then clarify federated identity and server checks, endpoint discovery, and privacy authorization. Done criteria and implementation boundaries are not specified beyond the desired cross-instance availability exchange.