Build a payment wallet

• Accounts hold a balance in integer minor units and a currency; topUp(accountId, amount, idempotencyKey) credits the balance
• transfer(fromId, toId, amount, idempotencyKey) debits one account and credits the other as one atomic step; insufficient funds, same-account transfers, non-positive amounts and unknown accounts are rejected
• Repeating a call with the same idempotency key returns the original result and does not apply it again, even when the first call is still in progress
• Every operation writes entries to an immutable transaction history; history(accountId, { limit, cursor }) returns them newest first, and balance equals the sum of entries
• Concurrent transfers touching the same accounts never produce a negative balance or a lost update, including opposite transfers between two accounts
• Total money in the system is conserved: a test transfers randomly between accounts concurrently and checks that the sum is unchanged

Only after every requirement works.

• Holds and captures (authorise now, settle later)
• Daily limits and fraud rules as pluggable checks
• Multiple currencies with explicit conversion