feat: add Google OAuth login with NextAuth

[not-your-average-devil]

What does this PR do?

Adds Google OAuth login functionality using NextAuth.

Fixes #1952

Type of change

• New feature (non-breaking change which adds functionality)

How should this be tested?

• Set up environment variables GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET
• Start the server locally
• Go to /api/auth/signin and verify Google login works

(Note: Only basic manual testing was performed locally)

Mandatory Tasks

• Make sure you have self-reviewed the code. A decent size PR without self-review might be rejected.

Checklist

• I have read the contributing guide
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have checked if my PR needs changes to the documentation
• I have checked if my changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have checked if new and existing unit tests pass locally with my changes

[vercel]

@not-your-average-devil is attempting to deploy a commit to the Piyush Garg's projects Team on Vercel.

A member of the Team first needs to authorize it.

[not-your-average-devil]

Hi! Just a quick note — the only failing check is from Vercel and it says “Authorization required to deploy.”
The Google OAuth feature is working and was manually tested locally. Let me know if you'd like any changes from my end.