Upsert the dunning schedule for this business

Create or replace the retry schedule. retries is an ordered list of steps, each with a dayOffset (days after firstFailedAt), a retryPayment flag, and an optional emailTemplate (dunning_first_notice | dunning_second_notice | dunning_final_notice). Steps must be strictly increasing by dayOffset. Set isActive: false to disable automatic retries without losing the schedule — manual retries via POST /collections/{id}/retry still work.