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/templates/base.html

@@ -592,6 +592,10 @@
                 <i class="fas fa-users"></i>
                 <span>Destinataere</span>
             </a>
+            <a class="sidebar-link" href="{% url 'stiftung:onboarding_einladung_liste' %}">
+                <i class="fas fa-user-plus"></i>
+                <span>Onboarding</span>
+            </a>
             <a class="sidebar-link" href="{% url 'stiftung:foerderung_list' %}">
                 <i class="fas fa-gift"></i>
                 <span>Foerderungen</span>