Design an internal service for a ride-hailing company that maintains a driver queue for each pickup area, such as an airport pickup zone or staging lot. When a driver enters the area, the driver should be added to that area's queue. When the driver leaves the area, goes offline, or becomes ineligible, the driver should be removed. Internal consumers should be able to query the current queue for a given pickup area and see the ordering of drivers. Discuss the API design, data model, event ingestion, queue ordering rules, consistency model, scalability, fault tolerance, and observability.