"feat: Refactor database connection in app.py and myp.db-shm"

backend/app/app.py

@@ -1673,7 +1673,7 @@ def api_admin_system_health():
         # 1. Datenbank-Schema-Integrität prüfen
         try:
             # Test verschiedene kritische Tabellen und Spalten
-            db_session.execute("SELECT COUNT(*) FROM guest_requests WHERE duration_minutes IS NOT NULL")
+            db_session.execute(text("SELECT COUNT(*) FROM guest_requests WHERE duration_minutes IS NOT NULL"))
             schema_integrity = "OK"
         except Exception as e:
             critical_errors.append({
@@ -1697,7 +1697,7 @@ def api_admin_system_health():
         missing_columns = []
         for table, column in schema_checks:
             try:
-                db_session.execute(f"SELECT {column} FROM {table} LIMIT 1")
+                db_session.execute(text(f"SELECT {column} FROM {table} LIMIT 1"))
             except Exception:
                 missing_columns.append(f"{table}.{column}")