Security Fix: Add access control to set_risk_tier function

[omolobamoyinoluwa-max]

• Add initialize(admin) function to set trusted admin address once
• Add get_admin() function to retrieve current admin
• Modify set_risk_tier to require caller to be admin OR the user themselves
• Add comprehensive unit tests for all authorization paths
• Fix security vulnerability where any address could overwrite any user's risk score

This addresses the security issue that blocked mainnet deployment and ensures downstream protocols can trust the on-chain credit score.

Closes #50

[vercel]

@omolobamoyinoluwa-max is attempting to deploy a commit to the mericcintosun Team on Vercel.

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[mericcintosun]

Thanks a lot for the contribution! 🙏

Closing this because the branch commits build artifacts (files under risk_score/target/** and/or **/node_modules/**) — that adds tens of thousands of generated files to the repository, which we can't merge.\n\nThe underlying change may well be valuable. Please feel free to resubmit a clean branch that:\n- adds risk_score/target/ and node_modules/ to .gitignore\n- contains only the source changes (no generated/build files, no full lockfile rewrites)\n\nAppreciate the effort!