Design a real-time auction feature for a large social photo and video app. A creator can attach an auction to a post. Viewers can place bids, and everyone watching the auction should see the current highest bid with low latency. The system must handle normal auctions as well as celebrity auctions where a large number of users bid at the same time. Cover functional requirements, non-functional requirements, APIs, data model, high-level architecture, bid ordering and concurrency control, real-time highest-bid updates, scaling for hot auctions, and failure handling. Pay special attention to why pessimistic locking or single-writer serialization may be preferable to optimistic locking for highly contended celebrity auctions.