logging in to other apps now works! made a quick demo using better-auth (on a separate website) to test it.

implemented OAuth2 code exchange and user info endpoints. I wrote them using orpc, so they have proper input validation, and as a bonus I can generate OpenAPI docs too ^-^

also added a ‘return_to’ query parameter so if you’re not logged in, it asks you to log in first, then redirects you back where you were.