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

.taskmaster/state.json

@@ -0,0 +1,6 @@
+{
+  "currentTag": "master",
+  "lastSwitched": "2025-09-08T21:19:13.841Z",
+  "branchTagMapping": {},
+  "migrationNoticeShown": false
+}