feat : Implement OAuth

[Krishna-kg732]

Implement OAuth Authentication

Add OAuth-based login (e.g., Google/GitHub) to enable users to sign in without creating credentials.

Scope:

• Integrate OAuth 2.0 (Authorization Code Flow)
• Add backend routes for login and callback
• Fetch user profile and create/link user in DB
• Issue session (JWT/cookie) after successful auth
• Add frontend “Continue with …” button

Acceptance Criteria:

User can log in via OAuth
New users are auto-created
Existing users are correctly linked
Session persists and logout works

Checklist

• Set up OAuth app (Google/GitHub) and obtain client ID/secret
• Configure redirect/callback URL
• Implement /auth/login (redirect to provider)
• Implement /auth/callback (handle code → token exchange)
• Fetch user profile from provider
• Create or link user in database
• Add OAuth fields (provider, provider_id, etc.)
• Generate session (JWT/cookie) after login
• Add frontend “Continue with …” button
• Handle success + error states in UI
• Implement logout flow
• Secure secrets via environment variables
• Validate state parameter (CSRF protection)
• Test full auth flow end-to-end

/good first