fix: Korrigiere Nginx PHP-FPM Upstream-Konfiguration für Staging

- Fix upstream configuration in staging-nginx container
- Verbessere sed-Befehle zur automatischen Korrektur der PHP-FPM Upstream-Definition
- Behebt 502 Bad Gateway Fehler durch korrekte Verbindung zu staging-app:9000

deployment/ansible/inventory/production.yml

@@ -11,6 +11,7 @@ all:
       vars:
         # Note: Centralized variables are defined in group_vars/production.yml
         # Only override-specific variables should be here
-        
+        # Override system_* defaults here when Wartungsfenster abweichen
+
         # Legacy compose_file reference (deprecated - stacks now use deployment/stacks/)
         compose_file: "{{ stacks_base_path }}/application/docker-compose.yml"