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/nachweis_board.html

@@ -13,6 +13,13 @@
                 Nachweis-Board {{ jahr_filter }}
             </h1>
             <div class="d-flex gap-2">
+                <form method="post" action="{% url 'stiftung:batch_nachweis_aufforderung_senden' %}">
+                    {% csrf_token %}
+                    <input type="hidden" name="jahr" value="{{ jahr_filter }}">
+                    <button type="submit" class="btn btn-primary" onclick="return confirm('Nachweis-Aufforderungs-E-Mails für alle offenen Nachweise {{ jahr_filter }} versenden?')">
+                        <i class="fas fa-paper-plane me-2"></i>Aufforderungen senden
+                    </button>
+                </form>
                 {% if overdue_count > 0 %}
                 <form method="post" action="{% url 'stiftung:batch_erinnerung_senden' %}">
                     {% csrf_token %}
@@ -126,6 +133,14 @@
                                                 {{ nachweis.get_completion_percentage }}%
                                             </a>
                                         </div>
+                                        {% if nachweis.status == 'offen' or nachweis.status == 'teilweise' or nachweis.status == 'nachbesserung' %}{% if row.destinataer.email %}
+                                        <form method="post" action="{% url 'stiftung:nachweis_aufforderung_senden' nachweis_pk=nachweis.pk %}" class="mt-1">
+                                            {% csrf_token %}
+                                            <button type="submit" class="btn btn-outline-primary btn-xs" style="font-size:0.65rem;padding:1px 5px;" title="Upload-Link per E-Mail senden" onclick="return confirm('Upload-Aufforderung an {{ row.destinataer.email }} senden?')">
+                                                <i class="fas fa-paper-plane"></i>
+                                            </button>
+                                        </form>
+                                        {% endif %}{% endif %}
                                     {% else %}
                                         <span class="text-muted small">–</span>
                                     {% endif %}