Double Refund

Task

A support ticket says a customer was refunded more than they paid. It happens when a support agent clicks Refund twice within a minute. The refund service has a controller, a service and two repositories over a small fake database. Reproduce the ticket, find the layer where the rule breaks, and fix it there.

Requirements

1. handle(charges, requests) loads the charges into a fresh database, replays the requests in order, and returns one response per request. A charge is {id, amount} with a unique id and a positive amount in integer cents (ValueError containing charge otherwise). Do not change the arguments.
2. {op: "refund", charge, amount, at} creates a refund and returns {status: 201, refund}. A refund is {id, charge, amount, status, created_at}. Ids run rf_1, rf_2 and so on. status starts as "pending". created_at is the request's at, in seconds (0 when left out).
3. A customer must never get back more than they paid for a charge. A refund that would allow it returns {status: 422, error: "over_refund"} and is not stored. A failed refund returns no money, so it does not count against the charge. Other refusals: an amount that is not a positive integer is 400 invalid_amount, an at that is not a non-negative integer is 400 invalid_time, and an unknown charge is 404 charge_not_found.
4. {op: "settle", refund} and {op: "fail", refund} record the payment processor's result for a pending refund and return {status: 200, refund}. An unknown refund is 404 refund_not_found. A refund that is no longer pending is 409 not_pending.
5. {op: "summary", charge} returns {status: 200, summary}. summary is {charge, amount, pending, settled, failed, refundable}: the refund totals by status, and how much of the charge can still be refunded. Any other op raises ValueError containing op.