Start the passwordless authentication flow

Starts the passwordless authentication flow by sending an email to the user with a sign-in link. When the user clicks the link, they are directed to GET /blaize/token-exchange?token=..., which authenticates them automatically. IMPORTANT: Passwordless authentication requires the user's email to be verified. This means the user must first receive a separate verification email and click the link to confirm their email address. Verification can be triggered by sending a POST request to the appropriate endpoint.