Mint a chat session for streaming

Runs chat.createGuestSession (public — guests chat without an account, critical decision #4). Returns { publicId, sessionToken }; the id + token pair rides the POST /api/chat stream body, and the one-time token later authorizes POST /v1/chat/claim-guest-session after sign-in. Signed-in callers get sessionToken: null — the minted session is already theirs. Lazy server-side: no row is persisted until the first message lands. The procedure takes no input, so this route accepts no request body.