Send the signup Slack card for the authenticated user.

Takes the user's identity from the JWT and everything else from the database: the caller supplies a signup_type hint, which classify_signup drops wherever the row says otherwise. A row that says nothing cannot outrank anything, and every account's row is blank for the window between signup and onboarding - which is the window this endpoint is called in, and the whole reason the hint exists. So the hint is bounded rather than powerless: SignupType is the only vocabulary it can speak, and any account with a role or a journey recorded gets the card its row describes.