[Feature] Sync attendees in calendar app with answers in attendance app

[flow10]

When a event is linked between the calendar app and the attendance app (because on event creation in the attendance app you imported an event from the calendar) it would be handy, if also the attendees would match in both apps.
So when people accept a date in the attendance app this would be reflected in the calendar app and the other way around.

[luflow]

Thanks for the idea! After looking into it, we won't implement this as a full two-way attendee sync — but we want to solve the underlying need. Reasoning:

• Calendar attendees are email-based, attendance responses are Nextcloud-user- (and guest-) based — the mapping is not always unambiguous.
• Writing ATTENDEE/PARTSTAT changes into events goes through the CalDAV scheduling stack and can trigger iTIP invitation/update emails to all attendees — a real side-effect risk on every response change.
• The app's response model is richer than PARTSTAT (yes/no/maybe + comments, guests, check-in, deadlines, response history). A bidirectional sync onto the flat calendar model would create two competing sources of truth, which is exactly the kind of state that produces confusing bugs.

What we plan instead: #70 has been accepted (create calendar events from the app and keep them synced two-way for the core fields). As a follow-up to that, the app could maintain an automatically updated response summary in the linked event (e.g. "12 attending, 3 declined, 2 maybe" in the description) — so the "who is coming" information is visible in the calendar without duplicating per-person state.

Would that cover your use case, or is there a scenario where you specifically need the individual attendee status inside the calendar event? Marking this as blocked by #70 for now.

[flow10]

Thanks for considering. I think this solution makes sense at the moment.