Add LinkedIn OAuth 2.0 authentication

[rxmox]

Summary

Implements LinkedIn OAuth 2.0 authentication flow, allowing users to sign up and log in using their LinkedIn accounts alongside the existing email/password authentication.

Changes

• User Model: Added OAuth fields (linkedinId, linkedinUrl, profilePhoto, authProvider), made passwordHash optional for OAuth users
• LinkedIn OAuth Utility: Created helper functions for authorization URL generation, token exchange, and profile fetching
• Auth Controllers: Added linkedinAuth and linkedinCallback endpoints with CSRF state token protection
• Auth Routes: Added GET /api/auth/linkedin and GET /api/auth/linkedin/callback
• Server Validation: Added startup validation for LinkedIn environment variables
• Dependencies: Added axios for HTTP requests to LinkedIn API

Email Conflict Handling

If a LinkedIn email matches an existing password-based account, returns error asking user to login with password instead (prevents unauthorized account linking).

Security Features

• CSRF protection using JWT-signed state tokens (5-minute expiration)
• OAuth users attempting password login are redirected to use LinkedIn
• Pre-save validation ensures local auth users have passwords

Testing

• TypeScript compiles successfully
• Server starts with environment validation
• CSRF protection verified (invalid state → 401)
• LinkedIn redirect URL generation confirmed

Environment Variables Required

LINKEDIN_CLIENT_ID=<app-client-id>
LINKEDIN_CLIENT_SECRET=<app-client-secret>
LINKEDIN_CALLBACK_URL=http://localhost:4000/api/auth/linkedin/callback
FRONTEND_URL=http://localhost:19006

Note: @Lujarios pls add http://localhost:4000/api/auth/linkedin/callback to the LinkedIn app's authorized redirect URLs for testing.

[rxmox]

Please don't merge yet, there are a thing or two I still need to confirm redirect URL is added

[rxmox]