How can I obtain cookies for a user or a 'user-api-key' without requiring user interaction or browser redirects?

Oh cool!

There’s no HTTP API endpoint for creating UserApiKey objects for other users. The /user-api-key endpoint only creates keys for the logged-in user

Using MessageBus would be handy, but I’m not sure how to get around the authentication issues. You might need to do polling and/or rely on webhooks for when PMs are updated.