fix(backend): validate proof registration tiers

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## Description
<!-- Provide a concise summary of the changes made in this PR -->

Closes #<!-- Replace with issue number, e.g. Closes #1 -->

## Type of Change
- [ ] `feat`: New feature
- [ ] `fix`: Bug fix
- [ ] `docs`: Documentation update
- [ ] `refactor`: Code refactoring
- [ ] `test`: Test suite update

## Workspaces Impacted
- [ ] Frontend
- [ ] Backend
- [ ] Soroban Smart Contracts (`/contracts`)
- [ ] Noir ZK Circuits (`/circuits`)
- [ ] DevX / Tooling / Docs

## Checklist & Verification

- [ ] I have verified all build, lint, and test commands pass locally for affected workspaces.
- [ ] I have followed the Conventional Commits format for my commit messages.
- [ ] **Screenshots:** (Required if UI/Frontend changed) Attached below or marked N/A.
- [ ] **Security Notes:** (Required if auth, contracts, keys, or ZK circuits changed) Detailed below or marked N/A.

### Screenshots (if applicable)
<!-- Attach screenshots or GIFs showing UI changes -->

### Security Notes (if applicable)
<!-- Explain any security considerations, cryptographic assumptions, or contract state impacts -->

### Test Evidence
```bash
<!-- Paste terminal output showing passing test results -->
closes #19 

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