Two hundred seats, five thousand buyers

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Scenario

A concert goes on sale at noon. There are exactly 200 seats. About 5000 people will have the page open in the minute before the drop, and most of them will click Buy within the same two seconds. Checkout ends at an external payment provider that takes its time.

Requirements

exactly 200 seats can be sold; overselling even by one is unacceptable
about 5000 buyers arrive within the same two seconds
a seat is held for 10 minutes while the buyer checks out, then released if unpaid
the payment provider answers in 2-20 seconds and declines or errors on about 1% of charges
the sale page answers in under 300 ms at peak
a buyer who is too late gets a clear answer, not a timeout

Constraints

three engineers, no dedicated ops
the payment provider is fixed and cannot be changed
selling fewer than 200 seats because a hold expired is acceptable

Hints

0/6 requirements ticked