[GrantFox][High] Add optimistic concurrency for credit-line mutations

[greatest0fallt1me]

Summary

Add optimistic concurrency for credit-line mutations

Why this matters

Concurrent draw, repay, or limit updates can overwrite each other without detection.

Scope

Add version checks or row locking, conflict responses, and retry guidance for callers.

Acceptance criteria

• Concurrent writes cannot silently overwrite state.
• Conflicts return a stable retryable code.
• Successful updates advance the version exactly once.
• Integration tests exercise competing transactions.

Validation

Add regression coverage for existing behavior, failure modes, authorization boundaries, and compatibility. The implementation must pass the repository CI checks.

Non-goals

• Typo-only, formatting-only, or documentation-only changes.
• Unrelated refactors or dependency upgrades.
• Weakening existing security, authorization, CI, or production safeguards.

Contributor application

Before implementation, comment with relevant experience, a 1–4 bullet approach, and an estimate for opening the first draft PR. Wait for maintainer assignment before coding.

PR requirements

Use a feature branch, include Closes #<issue-number>, check every acceptance criterion, link criteria to code/tests, explain security and failure-mode considerations, and pass CI.

Reward-readiness

This is a substantive GrantFox campaign issue. Merge and CI success do not by themselves guarantee reward eligibility; final reward-readiness is determined by campaign review.

[arisu6804]

Please consider assigning this issue to me, as I’m prepared to contribute wherever it’s needed, whether that involves protocol logic, interface creation, service-layer development, visual implementation, or cross-component coordination.

[grantfox-oss]

🦊 GrantFox — @arisu6804 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #302)
2. Your PR will be reviewed by the Creditra maintainers

Good luck! Track your progress on GrantFox.

[arisu6804]

I’m taking this issue on. Approach:\n- require an explicit expected version at the HTTP boundary;\n- keep compare-and-set atomic in each repository;\n- pass the pre-flight version through draw and repay writes;\n- return stable retryable conflict details and add unit/integration-oriented coverage plus retry guidance.\n\nThe implementation branch is pushed and ready for review.