Backend: Implement optimistic locking on credit line updates to prevent lost writes

[greatest0fallt1me]

Description

Add optimistic locking (version column) to credit line records so concurrent updates do not silently overwrite each other. Return a 409 conflict with a clear error code when version mismatch occurs.

Requirements and context

• Must be secure, tested, and documented.
• Must define retry strategy for callers.

Suggested execution

git checkout -b feature/optimistic-locking-creditline

Implement changes

• Add version column and update repositories to check-and-increment.
• Integration tests for concurrent update conflict.
• Update OpenAPI error responses.

Test and commit

npm test

Example commit message

feat(db): optimistic locking for credit line updates with 409 conflicts

Guidelines

• Timeframe: 96 hours.