System Design: Account Management with Transfers, Payments, and Cashback

You are to design and implement an in-memory account management service that supports atomic balance updates, delayed cashback, and a leaderboard of top transfer spenders. Assume integer timestamps (e.g., seconds since epoch) and integer amounts in the smallest currency unit (e.g., cents).

Assumptions to make explicit:

• Amounts must be positive integers; operations with non-positive amounts fail.
• Timestamps are provided by the caller and should be used to determine due cashback.
• 2% cashback is computed as floor(0.02 × amount) (integer math: (amount * 2) / 100 ).
• Return Optional.empty() for any failure condition (e.g., missing account, insufficient funds, invalid input), unless otherwise stated.
• Transfer spending for topSpenders counts only outgoing transfer amounts from the transfer operation and excludes pay and cashback.

Implement the following operations with the specified semantics:

1. createAccount(int timestamp, String accountId)
   • Create a new account with 0 balance.
   • Return false if the account already exists; otherwise true.
2. deposit(int timestamp, String accountId, int amount)
   • Deposit amount into an existing account.
   • Return Optional.of(newBalance) on success.
   • Return Optional.empty() if the account does not exist or amount ≤ 0.
3. transfer(int timestamp, String sourceAccountId, String targetAccountId, int amount)
   • Transfer amount between two distinct existing accounts if the source has sufficient balance.
   • On success, return Optional.of(sourceNewBalance).
   • Return Optional.empty() if any precondition fails (missing accounts, identical accounts, amount ≤ 0, insufficient funds).
   • Increase only the source account's cumulative outgoing transfer total (used for topSpenders).
4. topSpenders(int timestamp, int n)
   • Return a List <String> of up to n accountIds with the highest cumulative outgoing transfer totals.
   • If fewer than n accounts exist, return all.
   • Use a deterministic tie-break (e.g., lexicographic accountId ascending).
5. pay(int timestamp, String accountId, int amount)
   • Deduct amount from the account if sufficient funds; return Optional.of(uniquePaymentId) on success.
   • Return Optional.empty() on failure (missing account, amount ≤ 0, insufficient funds).
   • Automatically schedule 2% cashback after 24 hours (86,400 seconds) from the provided timestamp.
   • The cashback credit must not affect topSpenders.
6. getPaymentStatus(int timestamp, String accountId, String paymentId)
   • Return Optional.of("IN_PROGRESS") if the cashback has not yet been applied.
   • Return Optional.of("CASHBACK_RECEIVED") after the cashback is applied.
   • Return Optional.empty() if the account or paymentId is invalid or mismatched.

Additional requirements:

• Ensure atomicity and correctness under concurrency (e.g., transfers must not lose or double-count funds).
• Ensure each call first processes any due cashbacks up to the provided timestamp.