Multi Provider OAuth Integration

[Dhirenderchoudhary]

Summary

Add OAuth login for Discord, Google, LinkedIn, GitHub — extends existing session-based auth. No Passport.js (conflicts with existing UserSessionService + breaks Controller→Service→Repo pattern).

Tech spec: https://www.notion.so/oauth-techspec-358aea5e3e0c80ba98cbf360959558a2

Scope

• OAuth utility layer: generate auth URL + exchange code → normalised profile (id, email, username, avatar). CSRF state param via cookie.
• User model: add discordId, googleId, linkedinId, githubId, authProvider enum. OAuth users get random UUID password.
• AuthService: processOAuthCallback(provider, code) — lookup by provider ID → email → else create user. Block if is_deleted.
• Controllers/Routes: replace Discord-only controllers/routes with generic GET /api/auth/:provider and /:provider/callback.
• MFA: check mfaEnabled on callback, route to existing MFA flow if enabled.

Env vars needed

GOOGLE_CLIENT_ID/SECRET/REDIRECT_URI, LINKEDIN_CLIENT_ID/SECRET/REDIRECT_URI, GITHUB_CLIENT_ID/SECRET/REDIRECT_URI