Send notifications when a user tries to book a already booked room/resource

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

Hi,

We use the room resource to book meeting rooms. When using the web client in nextcloud the users can only book a resource if it is available and the event in the resource calendar will accept the invitation (showing a checkmark in the users calendar). However if a user adds a resource via CalDAV (In apple Calendar for example) it is possible to book a resource that is already booked, leading to double bookings. Also when booking from CalDAV the event in the resource calendar will stay in the "Invitation will be sent" status and show a ? in the users calendars.

Describe the solution you'd like

When booking from CalDAV the resource should be automatically accepted if the resource is available. If the resource is already booked the system should decline it and send out a notification about it to the user to avoid double bookings.

Describe alternatives you've considered

No response

Additional context

No response

Start by tracing the repository's CalDAV resource-booking flow and how invitations are accepted or declined. Confirm how bookings from CalDAV differ from web-client bookings and where user notifications are handled. Done means available resources are accepted automatically, unavailable resources are declined, and the user is notified without allowing double bookings.