Create recurring requests

Create multiple recurring requests based on a recurrence pattern. Note: half_day='morning'/'afternoon' requires a leave type with leave_unit 'half_days' — on a full-day-only 'days' leave type it is rejected. Employment period: the series is created all-or-nothing, so a single occurrence that starts before employment_start_date or ends after employment_end_date rejects the whole series with 400 rather than skipping that occurrence. An occurrence straddling either boundary is rejected too, never truncated. The comparison is on calendar days, so both boundary dates are themselves bookable and a null boundary means open ended on that side.