fix(backend): validate identity tier on proof registration

[enliven17]

Problem

Proof registration accepts tier values without enforcing the protocol enum.

Scope

Implement one focused change in the backend workspace. Preserve current public behavior unless an acceptance criterion changes it.

Acceptance Criteria

• Accept only silent, source, or seal
• return a stable 400 error for invalid values
• keep stored values consistent with embed metadata
• add missing, valid, and invalid tests

Test Plan

• Add automated positive and negative tests for the changed behavior.
• Run all existing checks for the affected workspace.

Out of Scope

• Unrelated refactors, production deployment, or live credential changes.

Drips Wave

Complexity: trivial

Apply through Drips Wave and wait for assignment before coding. The PR must include Closes # and concrete test evidence.

[vincentokoye953-gif]

@vincentokoye953-gif has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @vincentokoye953-gif to this issue.

[drips-wave]

Congratulations, @vincentokoye953-gif! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @vincentokoye953-gif: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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