How can I do this with the least amount of code? Is there a platform-as-a-service that integrates Google sign-in and Stripe subscription billing?
Ideally I want to just include a line of javascript (provided by the platform) to my web pages which adds a 'Log in/Register' button in the top-right corner of my webapp - then I just check the user's subscription status on the server to enable the feature. I don't want to mess around with Google authentication tokens or Stripe webhook callbacks.
I'd look at Auth0, Firebase, and adjacent products for the auth part.
Stripe really does the heavy lifting with subscriptions, shouldn't be much work to implement.