Get a ZK proof for a risk assessment

Retrieve a ZK proof artifact (individual or team-coherence) by its proof id. Proofs are generated fire-and-forget after /risk/assess and /risk/assess/team and may take tens of seconds to complete. Authorization: JWT required; the proof is returned only after the risk service re-verifies that the assessment it proves belongs to the caller's billing account (resolved from the token, never the path), so a proof owned by another account (or absent) returns 404 and never another tenant's data.