Get an invitation sent to the signed-in user.

The full invitation, including the invitee's and inviter's email addresses and the role it grants. Only the invited recipient (the signed-in user whose email matches, without regard to case) may read it. An unknown id, an id that is not a UUID, an invitation that is no longer pending, and an invitation sent to someone else are all answered with the same 404, so this never confirms that an invitation exists to anyone but its recipient. Service accounts are refused with a 403 (service_account_forbidden).