Add OAuth integration for multiple providers and implement user linking

- Created OAuth credentials setup guide for Google, Microsoft, Keycloak, ORY, and ZITADEL.
- Added migration for oauth_identity table to store OAuth identities linked to users.
- Implemented OAuthIdentity model for managing OAuth identities in the database.
- Developed oauthService to handle OAuth login, user creation, and identity linking.
- Created OAuthCallbackView and OAuthUserCallbackView components for handling OAuth responses in the frontend.
- Added error handling and user feedback during the OAuth process.

backend/package.json

@@ -39,6 +39,7 @@
     "multer": "^2.1.1",
     "mysql2": "^3.20.0",
     "nodemailer": "^8.0.3",
+    "openid-client": "^6.8.4",
     "pg": "^8.20.0",
     "pg-hstore": "^2.3.4",
     "redis": "^5.11.0",