Create a repeating schedule (Slot Series) for a signup

Create the signup's Slot Series and generate its Occurrences — one time slot per Shift per date the rule runs — in one transaction. rule is the shift plan: daily and monthly carry genericShifts; weekly carries activeDays, keyed by weekday number (0 = Sunday), and a weekday absent from the map is a day the Series does not run. Each Shift is { start, end, label?, capacity?, question? } with start / end as HH:MM wall-clock times in the signup's timezone. rangeStart / rangeEnd are YYYY-MM-DD. capacity is each Occurrence's default and a Shift's own capacity overrides it. A signup has at most one Series: a second create is 409 conflict. A range over 3660 days, or one that would produce more than 2000 Occurrences, is 400 bad_request. Repeating schedules need a paid plan: an organization without one gets 402 upgrade_required and nothing is written. Generating dates notifies nobody.