Aggregate view: merge multiple calendars into a single composite calendar

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

In environments with many calendars (rooms, groups, users, resources), it is often necessary to create a single, aggregated view.

Examples:

• A parish wants to show “All rooms at location X” in one combined view.
• A school needs “All sports facilities” calendar.
• A municipality wants “All public events” calendar.

Currently, the only way to achieve this is:

• Share each calendar individually.
• Ask users to subscribe to all relevant calendars and manually overlay them.
• Or merge ICS files externally (via scripts or third-party tools).

This is cumbersome and error-prone. Non-technical users cannot easily manage such setups.

Describe the solution you'd like

A feature to define an aggregate calendar inside Nextcloud, which combines selected calendars into one read-only composite view.

• Admin or user can select which calendars should be included.
• The system creates a new virtual calendar that contains all events from the selected ones.
• The aggregate calendar can be shared or published like any normal calendar (including as ICS feed).
• Events retain original attributes (title, category, resource) but optionally can be anonymized (only “busy” info).

Describe alternatives you've considered

• Manual ICS merge scripts: technically possible but requires external tooling and maintenance.
• Subscribing to multiple calendars in clients: works but does not scale and is not shareable as one combined feed.
• Dashboard widgets: show multiple calendars, but not in one unified calendar.

Additional context

• This feature would be extremely valuable in parish/municipal settings, where different rooms and groups are managed separately but a single “public calendar” is needed for the website or notice board.
• It would also simplify life in schools, NGOs, or companies with multiple teams, as they could provide an “all-hands overview” calendar without manual effort.
• Many competing solutions (Exchange, Google Calendar, Groupware systems) already provide aggregate or category-based views.

No files, tests, or entry points are named. Start by mapping how calendars are selected, represented, shared, and published as ICS feeds, then determine how a read-only composite calendar would preserve or anonymize event attributes. Done means selected calendars appear as one shareable or publishable aggregate without manual external merging.