Feat/google_oauth

[Mkalbani]

closes #122

Google OAuth 2.0 Authentication

Implement Google OAuth 2.0 login with extensible provider structure for future integrations (GitHub, Facebook).

Changes:

• Install passport-google-oauth20, @nestjs/passport, passport, @nestjs/jwt, passport-jwt
• Configure GoogleStrategy with client credentials via environment variables
• Routes: GET /auth/google and GET /auth/google/callback
• Add authProvider enum field to User entity ('local' | 'google' | 'github')
• User service links Google ID to existing users by email or creates new users with profile data
• AuthService issues JWT token on successful authentication
• Modular design allows easy addition of GitHub, Facebook providers

Testing:

• Google login redirects and returns JWT token + user data
• Returning users linked by email; first-time users created with Google profile
• Both flows tested and functional