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/stiftung/administration.html

@@ -271,6 +271,12 @@
                                 <span>Dokumentenverwaltung</span>
                             </a>
                         </div>
+                        <div class="col-md-3 mb-3">
+                            <a href="{% url 'stiftung:vorlagen_liste' %}" class="btn btn-outline-primary w-100">
+                                <i class="fas fa-file-code d-block mb-2 fa-2x"></i>
+                                <span>Dokument-Vorlagen</span>
+                            </a>
+                        </div>
                     </div>
                 </div>
             </div>