Build an auction system

• A seller creates an auction for an item with a start price, a minimum bid increment, an optional reserve price and an end time; users place bids with placeBid(auctionId, userId, amount)
• A bid is accepted only if the auction is open, the bidder is not the seller, and the amount is at least the current highest bid plus the increment (or the start price for the first bid)
• The current highest bid and bidder are always consistent; bids arriving concurrently (including in the same tick or with awaits between steps) are applied one at a time, with the later lower bid rejected
• A bid in the last minute extends the end time (anti-sniping) by a configurable amount
• Closing is driven by an injectable clock: a closed auction rejects bids, and close(auctionId) determines the winner (highest bid at or above the reserve) or no winner
• Observers are notified of outbid, auction won and auction ended without a winner events, and the full bid history is available

Only after every requirement works.

• Proxy (automatic) bidding up to a maximum amount
• Sealed-bid and Dutch auction types as strategies
• Scheduled closing with timers and cleanup