Security: UTXO Stats Authorization

[MichaelSovereign]

Summary

This PR hardens the UTXO API by requiring administrative authorization to access detailed system statistics.

Changes

1. Authenticated Access: Updated the endpoint to require a valid or .
2. Information Disclosure Mitigation: Prevents unauthorized entities from gathering detailed intelligence about the UTXO set size, total value, and transaction volume, which could be used to facilitate targeted attacks or network analysis.
3. Defense in Depth: Aligns the UTXO statistics endpoint with the security standards used in other administrative modules of RustChain.

Closes #6460

[Scottcjn]

Closing as part of Tier 0 hard-ban cleanup — see #3074 / #3104 / #3169 for the documented incident chain. All MichaelSovereign PRs are closed unread per the Tier 0 contract. No review path; no future PRs from this account will be processed. (See feedback_michaelsovereign_tier0_2026-05-02.md.)