List reservable sites, zones, and retailers

Discovery for reservations: every reservable site, its zones, and the retailers each zone currently supports for your organization. Only retailers granted to your organization appear, so a new organization sees an empty list until Autolane grants its first retailers; an organization whose onboarding has not reached this environment gets 403 ORG_INACTIVE instead, never an empty list. Poll this endpoint to keep your picker current: it is the source of the zone_id and retailer_id you send when creating a reservation.