Unarchives an existing, currently archived contact for the
authenticated client.

Requires contact_id. Only affects contacts where
contact_archived_at IS NOT NULL - attempting to unarchive a
contact that is not archived returns a failure response.

On success:

• contact_archived_at is cleared (set to NULL).
• If the contact has an associated user (contact_user_id > 0),
  that user record is also unarchived (user_archived_at cleared).