Fix positive repayments for negative balances

[Keviro]

Closes #18

Summary

• only enforce the minimum-balance validation when a transaction reduces the receiver account balance
• allow positive transactions to accounts that are still below the currency minimum after the transaction
• keep negative balance-reducing transactions blocked so players cannot go further into debt

Validation

• Reviewed the existing transaction persistence flow: failed persistence rolls back, and transfer sender failures are still mapped from ReceiverInsufficientFunds to SenderInsufficientFunds.
• Manually checked use cases:
  • player with a negative balance receives money but remains negative: allowed
  • player with a negative balance receives enough money to become non-negative: allowed
  • player tries to send/pay money while that would put or keep their own account below the minimum: blocked as sender insufficient funds
  • direct negative transaction that would further reduce an account below minimum: blocked
  • receiver side of a normal transfer is positive: allowed, including when repaying debt

Notes

• I could not run the Gradle test suite locally in this environment because the container has no network access to clone/resolve the repository, so the validation above is based on code inspection and branch diff review.

[Copilot]

Pull request overview

This PR updates the microservice-side minimum-balance enforcement so that accounts with negative balances can receive positive transactions (repay debt) without being blocked, while still preventing transactions that would further reduce an account below the currency minimum.

Changes:

• Gate the minimum-balance validation behind a “balance-decreasing transaction” condition (amount < 0).
• Preserve existing rollback behavior by keeping the validation inside the same DB transaction flow.