Retained legacy exception: Configure automatic retry on the original campaign; a new resend is a different operation.

Enable the auto-retry policy for an existing broadcast campaign. Provide either retryDays (1–7 days from the broadcast end) or retryUntil (an explicit UTC deadline), not both. This is not a failed-recipient resend that creates a separate campaign. The campaign must be completed, the deadline must be after the campaign end and within seven days, and Business or Pro is required.