Late-bind a session pubkey to an envelope that was created without one

MCP create_signing_envelope creates envelopes whose session_pubkey_jwk_json is NULL (an AI agent has no IndexedDB to keep a private key in). The first browser to open the resulting signing_url generates a non-extractable ECDSA P-256 keypair and POSTs the public JWK here. The handler UPDATEs the envelope row iff session_pubkey_jwk_json IS NULL (write-once, enforced by both the WHERE clause AND by trg_envelopes_session_update from migrations 0010 + 0011). Subsequent protected requests then carry the standard x-fsig-session-* headers. NOT required when the envelope was created via POST /api/envelopes directly — that call already binds the session.