BYOAI key envelope

Parent: access-control

A visitor's own model key (BYOAI) never touches server storage or Redis. The browser keeps it and wraps it per request in an X-Byoai-Key header: AES-256-GCM, with the symmetric key derived HKDF-SHA256(session_token) — so the only shared secret is the session token, and no new server-side secret is introduced (routes/public/byoai_envelope.go). The backend decrypts in-memory and discards it right after the LLM call.