feat: oauth autoredirect support

[joshtgl]

Description

Adds optional automatic redirect from the sign-in page to the first configured OAuth provider when OAUTH_AUTO_REDIRECT=true.

Let me know if any other changes/updates are needed.

Demo

No UI screenshots included. Behavior change is:

• With OAUTH_AUTO_REDIRECT=true and at least one OAuth provider configured, visiting the sign-in page redirects immediately to the first configured OAuth provider.

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks
• Any AI code was thoroughly reviewed by me