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/env.example

@@ -19,3 +19,20 @@ DB_PASS=
 #
 # Optional (Defaults siehe utils/sequelize.js)
 # DB_CONNECT_TIMEOUT_MS=30000
+
+# OAuth / OpenID Connect
+# FRONTEND_URL muss auf die öffentliche Frontend-URL zeigen, damit der Provider nach dem Login
+# korrekt auf den Callback im SPA zurückspringen kann.
+# OAUTH_GOOGLE_CLIENT_ID=
+# OAUTH_GOOGLE_CLIENT_SECRET=
+# OAUTH_MICROSOFT_CLIENT_ID=
+# OAUTH_MICROSOFT_CLIENT_SECRET=
+# OAUTH_KEYCLOAK_ISSUER=
+# OAUTH_KEYCLOAK_CLIENT_ID=
+# OAUTH_KEYCLOAK_CLIENT_SECRET=
+# OAUTH_ORY_ISSUER=
+# OAUTH_ORY_CLIENT_ID=
+# OAUTH_ORY_CLIENT_SECRET=
+# OAUTH_ZITADEL_ISSUER=
+# OAUTH_ZITADEL_CLIENT_ID=
+# OAUTH_ZITADEL_CLIENT_SECRET=