v1.5.7 — GET /api/attestation-info discovery + fingerprint

Bank SIEM pipelines auto-hydrate the verifier's public key by hitting a single endpoint. Ed25519 mode returns public_key_pem + SHA-256 SPKI fingerprint (RFC 5280). HMAC mode exposes only mode + key_id. 5-min cache. Auditors cross-check the endpoint fingerprint against an out-of-band copy from procurement time to detect silent rotation. Tests: 548 → 556 (+8). All green.