Add request and SQL performance logging features to backend

- Implement request timing middleware in app.js to log slow requests and all requests based on environment variables.
- Enhance sequelize.js with optional SQL query timing and logging capabilities, allowing for performance monitoring of database queries.

backend/utils/sequelize.js

@@ -3,6 +3,25 @@ import dotenv from 'dotenv';
 
 dotenv.config();
 
+// Optionales Performance-Logging (aktivierbar per ENV)
+// - SQL_BENCHMARK=1: Sequelize liefert Query-Timing (ms) an logger
+// - SQL_SLOW_MS=200: Logge nur Queries ab dieser Dauer (wenn SQL_LOG_ALL nicht gesetzt)
+// - SQL_LOG_ALL=1: Logge alle Queries (auch ohne benchmark)
+const SQL_BENCHMARK = process.env.SQL_BENCHMARK === '1';
+const SQL_LOG_ALL = process.env.SQL_LOG_ALL === '1';
+const SQL_SLOW_MS = Number.parseInt(process.env.SQL_SLOW_MS || '200', 10);
+const sqlLogger = (sql, timing) => {
+    // Sequelize ruft logging(sql) oder logging(sql, timing) abhängig von benchmark auf.
+    if (!SQL_BENCHMARK) {
+        if (SQL_LOG_ALL) console.log(sql);
+        return;
+    }
+    const ms = typeof timing === 'number' ? timing : 0;
+    if (SQL_LOG_ALL || ms >= SQL_SLOW_MS) {
+        console.log(`🛢️  SQL ${ms}ms: ${sql}`);
+    }
+};
+
 // Validiere Umgebungsvariablen
 const dbName = process.env.DB_NAME;
 const dbUser = process.env.DB_USER;
@@ -26,6 +45,8 @@ const sequelize = new Sequelize(dbName, dbUser, dbPass, {
         timestamps: false,
         underscored: true  // WICHTIG: Alle Datenbankfelder im snake_case Format
     },
+    benchmark: SQL_BENCHMARK,
+    logging: sqlLogger,
 });
 
 const createSchemas = async () => {