Feature: Outlook does not treat Nextcloud Appointments .ics as a meeting invite (no Accept/Decline buttons)

[JohnsonJohnny]

Description:
When using Nextcloud Appointments, the confirmation email includes an attached .ics file.
In Microsoft Outlook (Desktop and Outlook Web), this .ics is not recognized as a real meeting invitation.
Instead, Outlook treats it as a normal calendar file that can only be imported/added manually.

As a result, the recipient does NOT see the standard Outlook UI actions:

• Accept
• Tentative
• Decline
  (and no automatic RSVP workflow)

This makes it hard to use Nextcloud Appointments with Outlook-based organizations, because users expect a "real meeting request" they can respond to directly in Outlook.

Observed behavior:

• Email contains an attached .ics file
• Outlook opens it as a calendar event / import action
• No "Accept / Decline" buttons are shown
• No RSVP is sent back

Expected behavior:
Outlook should detect the appointment as a proper meeting request, so the recipient can respond directly via:

• Accept / Tentative / Decline
  and Outlook should handle the RSVP workflow.

Why this happens (likely root cause):
The generated ICS uses:

• METHOD:PUBLISH
  which Outlook treats as a published calendar event, not an invitation (iTIP meeting request).

Additionally, the generated ICS does not include the recipient as an ATTENDEE with RSVP required.
Example from the exported ICS:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//IDN nextcloud.com//Appointments App | srgdev.com//EN
CALSCALE:GREGORIAN
METHOD:PUBLISH
BEGIN:VEVENT
SUMMARY:Meet XXX @ XXX Appointment
STATUS:CONFIRMED
DTSTART;TZID=Europe/Berlin:20260202T093000
DTEND;TZID=Europe/Berlin:20260202T100000
ORGANIZER;CN=XXX:mailto:XXX@XXX.de
ATTENDEE;CN=YYY;PARTSTAT=ACCEPTED;ROLE=REQ-PARTICIPANT:mailto:YYY@ZZZ.de
END:VEVENT
END:VCALENDAR

Notes:

• METHOD:PUBLISH prevents Outlook from showing invitation actions (Accept/Decline)
• ATTENDEE is set to the organizer itself and already marked as ACCEPTED
• The actual recipient is not listed as an ATTENDEE with RSVP=TRUE / NEEDS-ACTION

Suggested fix / improvement:
Generate iTIP-compatible invites for recipients by using:

• METHOD:REQUEST
• Add the recipient as ATTENDEE with:
  • RSVP=TRUE
  • PARTSTAT=NEEDS-ACTION
• Keep ORGANIZER as the person who is hosting the appointment

This should make Outlook treat the ICS as a meeting invite and show the Accept/Decline UI.

Environment:

• Nextcloud Appointments (server-side email with .ics attachment)
• Microsoft Outlook Desktop + Outlook Web
• Timezone: Europe/Berlin

Impact:
Recipients on Outlook cannot respond properly to appointments.
This breaks the standard scheduling workflow for many enterprise users.