fix: return conflict for duplicate verified member identities (CM-1133)

[skwowet]

Summary

• Return 409 Conflict when verifying a member identity that is already verified on another member.
• Preserve the db uniqueness invariant while avoiding public API 500 responses for expected identity conflicts.
• Document the new 409 response in the public OpenAPI spec.

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Note

Low Risk
Low risk: changes are localized to member identity verification error handling and OpenAPI documentation, with no schema or workflow changes beyond mapping a known DB uniqueness violation to a 409 response.

Overview
Prevents expected duplicate-verified identity collisions from surfacing as 500 errors by catching the DB unique-constraint violation during verifyMemberIdentity and returning a 409 Conflict (ConflictError) when an identity is already verified on another member.

Updates the public openapi.yaml spec for PATCH /members/{memberId}/identities/{identityId} to document the new 409 response and example payload.

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[Copilot]

Pull request overview

This PR updates the public member-identity verification endpoint to return a 409 Conflict when an identity being verified is already verified on a different member, and documents that new response in the public OpenAPI spec.

Changes:

• Add a conflict check before verifying a member identity to prevent “duplicate verified identity” situations.
• Return ConflictError (HTTP 409) instead of allowing a DB uniqueness violation to surface as a 500 in the expected conflict case.
• Document the new 409 response for the identity verification endpoint in openapi.yaml.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
backend/src/api/public/v1/members/identities/verifyMemberIdentity.ts	Adds a pre-check for already-verified identities on other members and throws ConflictError.
backend/src/api/public/openapi.yaml	Documents the new 409 Conflict response for the identity verification PATCH endpoint.

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