Replaces board-level notification overrides for the specified employee (defaults to the calling employee). Admins and users with EDIT permission on the employee may pass employee_id to update another employee's overrides. Event types listed in items override the global preference for this board. Event types not listed revert to the employee's global preference. Sending an empty items array clears all overrides (global preferences apply). Returns 404 if the employee is not a subscriber of the board.