Remove a lesson cover form

Requires permission: LessonCoverAdmin

Soft-deletes a lesson cover form by id: cancels the calendar events its assignments put on assignees' calendars, then sets Removed so the form stops appearing anywhere the module reads forms, without deleting the row. There is no un-remove endpoint. If any calendar event cannot be cancelled the form is left in place and the call fails, so it is never reported as removed while stale invites survive; retrying only re-attempts the events still outstanding.