Backend: Add consistent conflict detection and 409 responses for duplicate resources

[greatest0fallt1me]

Description

Standardize how the API detects and responds to duplicates (duplicate credit line open, duplicate risk evaluation create, duplicate webhook subscription). Return a consistent 409 error with a stable code and actionable details.

Requirements and context

• Must be secure, tested, and documented.
• Must avoid leaking sensitive identifiers in conflict details.

Suggested execution

git checkout -b feature/conflict-409-standard

Implement changes

• Add a shared ConflictError type and mapping to problem+json.
• Update repositories/services to throw conflict on unique constraint violations.
• Integration tests for representative conflict scenarios.
• Update OpenAPI error responses.

Test and commit

npm test

Example commit message

feat(api): consistent 409 conflict responses for duplicate resources

Guidelines

• Timeframe: 96 hours.

[shinzoxD]

Hi maintainers, I would like to take this issue if it is available.

I would add a shared ConflictError/409 mapping that produces a stable code and safe problem+json details, then wire duplicate-resource paths such as credit line open, risk evaluation create, and webhook subscription through that common path. I would avoid exposing sensitive/internal identifiers in details and add integration tests for representative duplicate cases and unique-constraint translation.

I will wait for assignment or GrantFox confirmation before opening a PR.