Fix environment variable handling for production deployment

- Update settings.py to support both ALLOWED_HOSTS and DJANGO_ALLOWED_HOSTS
- Add production CSRF_TRUSTED_ORIGINS for vhtv-stiftung.de
- Update env-template.txt with production variable examples
- Improve compatibility between development and production environments

env-template.txt

@@ -9,6 +9,11 @@ DB_PORT=5432
 DJANGO_SECRET_KEY=your-secret-key-here-change-in-production
 DJANGO_DEBUG=1
 DJANGO_ALLOWED_HOSTS=localhost,127.0.0.1
+
+# Production environment variables (alternative names for Docker Compose)
+# SECRET_KEY=your-production-secret-key-here
+# ALLOWED_HOSTS=your-domain.com,www.your-domain.com,localhost,127.0.0.1
+
 LANGUAGE_CODE=de
 TIME_ZONE=Europe/Berlin