OHTTP gateway key config

HPKE public key configuration for Oblivious HTTP. Fetch this, encapsulate your request to the returned publicKey, then POST the sealed bytes to the RELAY at https://relay.nightferry.net/relay — not to this host. Suite: DHKEM(X25519,HKDF-SHA256)/HKDF-SHA256/AES-128-GCM. Response encryption uses the HPKE export secret with info "robyn-ohttp-response-v1"; the response is 12-byte nonce || ciphertext || 16-byte GCM tag under AES-256-GCM. NOTE: this wire format is the OHTTP architecture, not RFC 9458 byte-compatible — the inner message is JSON, not RFC 9292 BHTTP.