Security: Beacon API Authentication & Integrity

[MichaelSovereign]

Summary

This PR addresses critical authentication bypass vulnerabilities in the Beacon Atlas API.

Changes

1. Cryptographic Signatures: Implemented which requires every state-changing request (claims, contract updates) to be signed by the agent's private key.
2. Replay Protection: Added a mandatory timestamp and signature window (5 mins) for all authenticated requests.
3. Authorization Checks: Added checks to ensure agents can only update contracts they are actually parties to.
4. Data Integrity: Prevents unauthorized bounty hijacking and reputation manipulation.

Closes #7432