Appointments: create per group

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

At the moment, appointments are tied to a single user.

For public links, such as planning a sales demo, the user is irrelevant. As long as the appointment is planned with a user that belongs to a group (e.g. has access to the chosen calendar), and that user is available.

Describe the solution you'd like

It should be possible to create appointments on a global or per-calendar level, rather than for a specific user.

When creating an appointment via the public link, a user will be chosen based on availability.

Describe alternatives you've considered

Store all links for all users in some frontend, and redirect to the appointments link of a random user. This does not guarantee that the chosen user is available, though.

Additional context

No response

Start with the public-link appointment creation flow and the existing user-specific appointment behavior, then trace how calendar access and user availability are handled. Done means appointments can be created globally or per calendar and the public link selects an available eligible user without relying on a random user.