Add Vorlagen editor, upload portal, onboarding, and participant import command

- Dokument-Vorlagen-Editor: create/edit/reset document templates (admin)
- Upload-Portal: public portal for Nachweis uploads via token
- Onboarding: invite Destinatäre via email with multi-step wizard
- Bestätigungsschreiben: preview and send confirmation letters
- Email settings: SMTP configuration UI
- Management command: import_veranstaltung_teilnehmer for bulk participant import

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/core/urls.py

@@ -8,6 +8,8 @@ from stiftung.views import home
 
 urlpatterns = [
     path("api/v1/", include("stiftung.api_urls")),
+    # Öffentliches Portal (kein Login erforderlich – tokenbasiert)
+    path("portal/", include("stiftung.portal_urls")),
     path("", include("stiftung.urls")),
     path("admin/", admin.site.urls),
     # Authentication URLs