feat(auth): add Google OAuth2 login (#292)

[deborahamoni0-prog]

Closes #292

---

Summary

Implements Google OAuth2 authentication as described in issue #292.

Acceptance Criteria

• ✅ OAuth2 config (GOOGLE_CLIENT_ID, GOOGLE_CLIENT_SECRET, GOOGLE_CALLBACK_URL)
• ✅ Login endpoint (GET /auth/google → redirect, GET /auth/google/callback → tokens)
• ✅ Account linking (existing email → links googleId)
• ✅ Profile sync (firstName, lastName, avatar kept in sync on each login)

Changes

• GoogleStrategy — Passport strategy using passport-google-oauth20, extracts googleId, email, firstName, lastName, avatar from Google profile
• GoogleAuthGuard — thin guard wrapping AuthGuard('google')
• AuthService.googleOAuthLogin — handles three cases:
  • New user → creates account (pre-verified, no password required)
  • Existing email → links Google account to existing user
  • Returning Google user → syncs profile fields
• Endpoints — GET /auth/google (redirect) and GET /auth/google/callback (returns access + refresh tokens)
• Schema — User.password made optional, googleId unique field added
• Migration — 20260422000001_add_google_oauth
• Dependencies — @nestjs/passport@10.0.3, passport-google-oauth20@2.0.0

Config

Add to .env:

GOOGLE_CLIENT_ID=your-google-client-id
GOOGLE_CLIENT_SECRET=your-google-client-secret
GOOGLE_CALLBACK_URL=http://localhost:3000/api/auth/google/callback

Verified

• npm run lint — 0 errors
• npm run build — passes
• npm test — 9/9 tests pass

[drips-wave]

@deborahamoni0-prog Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[deborahamoni0-prog]

done