Fix DMS entity assignment and Geschichte document linking

- DMS edit view: add Destinatär, Land, Pächter, Verpachtung dropdowns
  so documents can be assigned to entities after upload
- Geschichte: add M2M dokumente field on GeschichteSeite model
- Geschichte form: checkboxes to select/link Stiftungsgeschichte docs
- Geschichte detail: show linked documents in sidebar with download

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

app/templates/stiftung/geschichte/detail.html

@@ -120,12 +120,39 @@
     </div>
     
     <div class="col-lg-4">
+        {% if seite.dokumente.exists %}
+        <div class="card mb-3">
+            <div class="card-header bg-dark text-white">
+                <h6 class="mb-0"><i class="fas fa-landmark me-2"></i>Verknüpfte Dokumente</h6>
+            </div>
+            <div class="card-body p-0">
+                <div class="list-group list-group-flush">
+                    {% for dok in seite.dokumente.all %}
+                    <a href="{% url 'stiftung:dms_detail' pk=dok.pk %}" class="list-group-item list-group-item-action">
+                        <div class="d-flex justify-content-between align-items-center">
+                            <div>
+                                <div class="small fw-bold">
+                                    {% if dok.is_pdf %}<i class="fas fa-file-pdf text-danger me-1"></i>{% else %}<i class="fas fa-file text-primary me-1"></i>{% endif %}
+                                    {{ dok.titel|truncatechars:35 }}
+                                </div>
+                                <small class="text-muted">{{ dok.dateiname_original }} &middot; {{ dok.get_human_size }}</small>
+                            </div>
+                            <span class="btn btn-sm btn-outline-success" title="Herunterladen" onclick="event.preventDefault(); window.location='{% url 'stiftung:dms_download' dok.pk %}';">
+                                <i class="fas fa-download"></i>
+                            </span>
+                        </div>
+                    </a>
+                    {% endfor %}
+                </div>
+            </div>
+        </div>
+        {% endif %}
+
         <div class="card">
             <div class="card-header bg-secondary text-white">
                 <h6 class="mb-0"><i class="fas fa-list me-2"></i>Weitere Seiten</h6>
             </div>
             <div class="card-body">
-                <!-- This could be populated with other history pages -->
                 <p class="text-muted small">Navigation zu anderen Geschichtsseiten wird hier angezeigt.</p>
             </div>
         </div>