feat: send calendar event invitations by email or federation

- New endpoint POST /api/calendar/events/:id/email sends event
  invitations with an .ics attachment to external email addresses
  (rate-limited like room email invites)
- Create-event dialog gets an optional invitations section to invite
  people by email or federation address right away
- The "Invite remote" dialog on existing events now accepts email
  addresses in addition to federation IDs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/i18n/en.json

@@ -43,6 +43,14 @@
       "subject": "{appName} - Calendar invitation from {fromUser}",
       "intro": "You have received a calendar invitation from {fromUser}."
     },
+    "calendarEventInvite": {
+      "subject": "{appName} - Calendar invitation: {title}",
+      "title": "Calendar Invitation",
+      "intro": "{fromUser} has invited you to the following event:",
+      "joinButton": "Join Meeting",
+      "icsHint": "You can add this event to your calendar using the attached .ics file.",
+      "footer": "This invitation was sent via {appName}."
+    },
     "calendarDeleted": {
       "subject": "{appName} - Calendar event cancelled: {title}",
       "intro": "The following calendar event was deleted by the organiser ({fromUser}) and is no longer available:",