Events of subscribed calendars should show up in the free/busy window

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

Events of subscribed calendars can't be considered when arranging appointments using the free/busy window.

Nextcloud was recently introduced in our school (for teachers and students) and we plan to use Nextcloud Calendar for teachers. The combined class & substitution schedules are available as ICS calendar subscriptions. (Each teacher has his / her own ICS calendar URL. Used service: WebUntis) When planning appointments or meetings, it is crucial that class & substitution schedules are taken into account. Without knowing which colleague is teaching when, the great "free/busy" feature is of no use to us - sadly.

Describe the solution you'd like

I want events of subscribed calendars to show up in the free/busy window to be able to consider those when arranging appointments.

Describe alternatives you've considered

As the content of the subscribed calendars can change several times a day and sometimes in very short time intervals, importing the calendars by hand is not a valid option.

Additional context

No response

Start by tracing the free/busy window and the handling of subscribed calendar events described in the issue. Determine how subscription data is represented and what is needed for those events to appear in free/busy results; the work is done when changing subscribed calendars is reflected without manual imports.