feat(calendar): implement local calendar invitations with email notifications

- Added functionality to create, accept, decline, and delete local calendar invitations.
- Integrated email notifications for calendar event invitations and deletions.
- Updated database schema to support local invitations and outbound event tracking.
- Enhanced the calendar UI to display pending invitations and allow users to manage them.
- Localized new strings for invitations in English and German.

src/i18n/en.json

@@ -411,6 +411,8 @@
     "roomDeletedNotice": "This room has been deleted by the owner on the origin instance and is no longer available.",
     "calendarEvent": "Calendar Invitation",
     "calendarAccepted": "Calendar event accepted and added to your calendar!",
+    "localCalendarEvent": "Local Calendar Invitation",
+    "calendarLocalAccepted": "Invitation accepted – event added to your calendar!",
     "invitationRemoved": "Invitation removed",
     "removeInvitation": "Remove invitation"
   },
@@ -458,6 +460,11 @@
     "shareAdded": "User added to event",
     "shareRemoved": "Share removed",
     "shareFailed": "Could not share event",
+    "invitationSent": "Invitation sent!",
+    "invitationCancelled": "Invitation cancelled",
+    "invitationPending": "Invitation pending",
+    "pendingInvitations": "Pending Invitations",
+    "accepted": "Accepted",
     "sendFederated": "Send to remote",
     "sendFederatedTitle": "Send Event to Remote Instance",
     "sendFederatedDesc": "Send this calendar event to a user on another Redlight instance. The recipient must accept the invitation before the event appears in their calendar.",