[Feat] Issue-1082 Add multi-provider OAuth login for Discord, Google, LinkedIn, GitHub

[Dhirenderchoudhary]

Description

Closes #1082

Adds multi-provider OAuth login support for Discord, Google, LinkedIn, and GitHub following the OAuth tech spec.

What this does:

• OAuth utility layer (utils/oauth.js) — provider-agnostic module that builds authorization URLs, exchanges codes for tokens, and normalises all provider profiles into a single { id, email, username, avatar } shape
• AuthService (services/auth.js) — 3-step user resolution: lookup by provider ID → fallback to email (links provider) → create new verified user
• User model — added discordId, googleId, linkedinId, githubId fields (sparse + unique) and authProvider enum
• Generic controllers — replaced per-provider routes with parameterised GET /api/auth/:provider and GET /api/auth/:provider/callback
• CSRF protection — cryptographic state parameter stored in httpOnly cookie, validated on callback
• MFA integration — OAuth callback checks mfaEnabled and redirects to MFA flow with ?mfaRequired=true&from=oauth
• Frontend — OAuth buttons on sign-in form, error/MFA redirect handling in App.jsx
• Edge cases — no-email rejection, soft-deleted account rejection, duplicate prevention via email fallback, GitHub private email fallback via /user/emails

What type of PR is this? (Check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📄 Documentation Update
• 👨💻 Code Refactor
• 🔥 Performance Improvements
• ✅ Test
• 🛠️ CI/CD

Screenshots (if applicable)

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud