Create a payment schedule

Generates one item per period from the lease params. The generated rows are
the source of truth from here on — edit them individually (partial
first/last month, waived periods, rent changes).

Send an optional Idempotency-Key header (X-Idempotency-Key is also
accepted) to make retries safe: repeating the same key with the same body
within 24 hours returns the schedule created the first time instead of a
second one. The same key with a different body, or while the first request
is still being processed, returns 409.