Enable private resource bookings (hide event details, show only busy status)

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

Currently, when a user creates a private calendar event (in a calendar only visible to them) and adds a resource (room, car, device), the resource booking automatically becomes visible in the resource calendar to all users with access.
This means that the time and event appear publicly in the resource calendar, even though the user intended the booking to be private.
In administrative and organizational contexts, this prevents staff from booking resources for private appointments (e.g. car usage, meeting room, office space) without exposing the existence of those private appointments to colleagues.

Describe the solution you'd like

A private booking mode for resources.

• When a user marks an event as private, the corresponding resource entry should only block the resource as busy, without exposing any details (title, participants, description).
• Other users should only see the resource as unavailable for that time span, but not know who booked it or why.
• Ideally, this behavior can be configured on a per-resource basis by the administrator (e.g. "all bookings anonymized" vs. "respect event privacy").

Describe alternatives you've considered

• Manually setting the resource event to private after booking (inconvenient and error-prone).
• Using two calendars (one private, one resource-shared) and duplicating events – which is not user-friendly and leads to mistakes.
• External resource management tools that support free/busy anonymization – but this breaks the Nextcloud integration.

Additional context

In a parish/municipality setting, staff need to book resources (meeting rooms, parish cars, equipment) even for appointments that are not meant to be visible to everyone. Other users should only see that the resource is blocked at a certain time, not the private details.
This is a common requirement in organizational contexts (schools, NGOs, parishes, public administration).

The issue names no files, tests, or entry points. Start by locating the PHP resource-booking flow and its handling of private calendar events, then determine how resource entries expose event details and whether per-resource privacy configuration is already supported. Done means private bookings show only busy status while preserving the requested resource availability behavior.