Add lent archive export and import

Summary

Implemented versioned lent archive export and additive import with strict validation, atomic writes, ID remapping, frontend controls, cache invalidation, and test coverage.

Decisions

• Use nested repayments in a versioned JSON archive so lent histories round-trip losslessly.
• Make imports additive with new IDs instead of adding restore-specific conflict or idempotency storage.
• Normalize money as exact decimal strings and enforce archive size and record limits before persistence.