Initialize model associations in server setup: Import and invoke setupAssociations to ensure all models share the same instance, preventing EagerLoadingError and enhancing data integrity before loading the app and services.

backend/server.js

@@ -3,6 +3,11 @@ import './config/loadEnv.js'; // .env deterministisch laden
 import http from 'http';
 import https from 'https';
 import fs from 'fs';
+// Assoziationen sofort setzen, bevor app (und damit Services/Router) geladen werden.
+// So nutzen alle Modelle dieselbe Instanz inkl. Associations (verhindert EagerLoadingError).
+import setupAssociations from './models/associations.js';
+setupAssociations();
+
 import app from './app.js';
 import { setupWebSocket } from './utils/socket.js';
 import { syncDatabase } from './utils/syncDatabase.js';