System Design: Airbnb-Style Wallet with Ledger, Holds, FX, and Idempotency

Context

You are asked to design and implement an Airbnb-style wallet system that supports customer deposits, booking holds and captures, refunds, host payouts, multi-currency balances with FX conversion, and a double-entry append-only ledger. The system must be auditable, concurrency-safe, and idempotent.

Assumptions (explicit and minimal):

• Amounts are stored in minor units (integers) to avoid floating-point errors.
• Each user has a wallet with per-currency sub-accounts. Platform and hosts are also modeled as accounts.
• A double-entry ledger ensures that for every entry, debits equal credits; balances are derived from the ledger. For performance, a materialized balances table is maintained transactionally.
• Booking currency is used for hold and capture; users may convert between currencies within their wallet via an explicit convert operation.

Requirements

1. Create user accounts and allow deposits.
2. Support booking holds/authorizations and later captures at check-in.
3. Allow hold release on cancellation or expiry.
4. Process partial/full refunds.
5. Pay out hosts after platform fees and taxes.
6. Enforce spendable vs. total balance with no negative spendable balance.
7. Support multi-currency balances with FX conversion and rounding rules.
8. Ensure idempotent operations via client-supplied idempotency keys.
9. Guarantee concurrency safety for duplicate or overlapping requests.
10. Maintain an append-only auditable ledger and derive balances from it.
11. Provide reconciliation to detect inconsistencies.
12. Define persistence schema (tables or classes) and transaction isolation choices.
13. Expose clear APIs/methods and include a minimal main() to run sample scenarios.
14. Write unit tests for happy paths and edge cases (insufficient funds, expired holds, double-capture attempts, partial refunds, FX rate changes). Include core classes (Wallet, Account, Transaction, Hold, Capture, Refund, Payout, LedgerEntry, FXConverter), their relationships, method signatures, and analyze time/space complexity of critical operations.