Add "Modify Event" Permission for Shared Events

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

Description:
I would like to request a new feature for Nextcloud Calendar that allows shared event participants to modify the event, similar to Google Calendar's "Modify event" permission.

Currently, when sharing a calendar or an individual event, users can either have full access (owner-level) or read-only access. However, in many collaborative environments, it would be beneficial to allow invited users to make modifications to a shared event without giving them full control over the entire calendar.

For example, in a team setting:

• A manager creates a meeting event and shares it with team members.
• Team members should be able to adjust the time, location, or description if needed.
• However, they should not be able to delete the event entirely or modify unrelated events in the calendar.

Describe the solution you'd like

Introduce a new permission setting: "Modify Event" that allows invitees to:
✅ Edit event title, description, date, time, and location.
✅ Add or remove participants (optional, based on permissions).
✅ Modify event recurrence settings.
❌ Not delete the event (unless they are the owner).
❌ Not modify other events in the calendar.

This permission could be added when sharing an event or calendar, allowing more flexibility in collaboration.

Describe alternatives you've considered

• Add a new "Modify Event" checkbox in the sharing settings of an event.
• Implement role-based access control (RBAC) for event-specific permissions.
• Ensure compatibility with CalDAV clients (e.g., Thunderbird, Apple Calendar, DAVx⁵).

Additional context

Adding this feature would improve Nextcloud Calendar’s usability in collaborative environments while keeping control and security intact. I appreciate the Nextcloud team’s efforts and would love to see this enhancement in future releases!

Thank you for considering this request. 🙌

Review the event and calendar sharing settings, then check the CalDAV compatibility points for Thunderbird, Apple Calendar, and DAVx⁵. Define the exact boundaries for editing fields, participants, recurrence, deletion, and unrelated events before implementation. Done means the permission works consistently for shared events and calendars without granting owner-level access.