Feature: Add OAuth2 support for third-party login

[wistfulvariable]

We should support Google and GitHub OAuth2 login in addition to username/password.\n\n## Requirements\n- Google OAuth2 sign-in\n- GitHub OAuth2 sign-in\n- Link OAuth accounts to existing username/password accounts\n- Store refresh tokens securely

[wistfulvariable]

I've started looking into the Google OAuth2 flow. We'll need to register the app in Google Cloud Console first. Should I create a separate branch for this?

[github-actions]

Benchmark results: Google OAuth2 adds ~200ms latency to login flow. GitHub OAuth adds ~150ms. Both within acceptable range for auth endpoints.

[github-actions]

Benchmark results: Google OAuth2 adds ~200ms latency to login flow. GitHub OAuth adds ~150ms. Both within acceptable range for auth endpoints.

[github-actions]

Benchmark results: Google OAuth2 adds ~200ms latency to login flow. GitHub OAuth adds ~150ms. Both within acceptable range for auth endpoints.