Calendar: cancel events for guests, apply organizers' cancellations, invitation settings

Deleting an event the user organizes, with other guests, asks "Would you like
to inform the participants that this event is canceled?" (Yes, inform them and
delete / No, just delete / Cancel; for a recurring one after which occurrences).
Yes sends each guest an iTIP CANCEL from the organizer's mailbox — of one
occurrence via RECURRENCE-ID — or, for "this and following", the shortened
series. Invitations now carry their RRULE, so recurring events repeat for
guests too.

An organizer's cancellation is applied as the email arrives (message.new hook)
or, for older mail, when the card is opened — always, whatever the settings:
only from that event's organizer and not older than the calendar's copy; one
occurrence becomes an EXDATE. The card says "Event was removed from calendar".

Settings → Calendars → Invitations now work, defaulting to "Do not
automatically update my calendar": "Only update events that I have already
added" applies the organizer's changes (keeping the user's answer), "Add new
events…" also adds new invitations, unanswered — from contacts or anyone — to
the chosen calendar, and "mark as read and file" archives such messages. With
no automatic updates, a changed invitation says so on its card.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/api/settings.py

@@ -42,7 +42,8 @@ class SettingsOut(BaseModel):
     default_calendar: str = ""  # calendar id new events default to (blank => first)
     default_reminders_timed: list[ReminderPref] = []  # defaults for timed events
     default_reminders_allday: list[ReminderPref] = []  # defaults for all-day events
-    invitation_policy: str = "add_update"  # how emailed events update the calendar
+    # How emailed events update the calendar: none | update_only | add_update.
+    invitation_policy: str = "none"
     invitation_from: str = "contacts"  # whose emailed events auto-add (contacts|anyone)
     invitation_calendar: str = ""  # calendar id emailed events land in (blank => default)
     invitation_file: bool = False  # also mark the invite message read + filed
@@ -119,7 +120,7 @@ def _settings_out(user: User) -> SettingsOut:
         default_calendar=str(s.get("default_calendar", "")),
         default_reminders_timed=_reminder_list(s.get("default_reminders_timed")),
         default_reminders_allday=_reminder_list(s.get("default_reminders_allday")),
-        invitation_policy=str(s.get("invitation_policy", "add_update")),
+        invitation_policy=str(s.get("invitation_policy", "none")),
         invitation_from=str(s.get("invitation_from", "contacts")),
         invitation_calendar=str(s.get("invitation_calendar", "")),
         invitation_file=bool(s.get("invitation_file", False)),