fix: handle member identity unique constraint conflicts

[skwowet]

Summary

• Handle unique constraint conflicts when creating member identities in the public API.
• Convert duplicate identity DB errors into 409 Conflict responses instead of unhandled 500 errors.
• Remove the redundant pre-insert identity existence check and rely on DB constraints as the source of truth.

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Note

Low Risk
Low risk, localized change to the public member identity create endpoint that maps known DB uniqueness violations to 409 Conflict instead of surfacing 500 errors.

Overview
Updates the public POST member identity creation flow to rely on DB uniqueness constraints rather than a pre-insert existence check.

On insert, it now catches specific unique-index violations (duplicate identity on the same member, or an identity already verified on another member) and returns 409 Conflict with clearer messages, while rethrowing unexpected errors.

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[github-actions]

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[Copilot]

Pull request overview

Updates the public API member identity creation endpoint to rely on database uniqueness constraints and convert specific unique-constraint violations into 409 Conflict responses (instead of bubbling up as 500 errors).

Changes:

• Removed the pre-insert “identity existence” lookup and relies on DB constraints as the source of truth.
• Added constraint-name-based handling to translate duplicate identity / duplicate verified identity inserts into ConflictError (409).

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