Fix email-Destinatär document linking and add email delete

- When manually assigning an email to a Destinatär, also update
  associated DokumentLink records so attachments appear in the
  Destinatär's Dokumente tab
- Add email delete functionality (view, URL, buttons in list and detail)

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

app/templates/stiftung/email_eingang/detail.html

@@ -211,7 +211,7 @@
         </div>
 
         <!-- Metadaten -->
-        <div class="card">
+        <div class="card mb-4">
             <div class="card-header"><i class="fas fa-info-circle me-2"></i>Metadaten</div>
             <div class="card-body">
                 <dl class="row mb-0 small">
@@ -222,6 +222,23 @@
                 </dl>
             </div>
         </div>
+
+        <!-- Löschen -->
+        <div class="card border-danger">
+            <div class="card-header text-danger">
+                <i class="fas fa-trash-alt me-2"></i>E-Mail löschen
+            </div>
+            <div class="card-body">
+                <p class="small text-muted mb-2">Diese E-Mail unwiderruflich aus dem System entfernen.</p>
+                <form method="post" action="{% url 'stiftung:email_eingang_delete' eingang.pk %}"
+                      onsubmit="return confirm('E-Mail wirklich löschen?');">
+                    {% csrf_token %}
+                    <button type="submit" class="btn btn-outline-danger w-100">
+                        <i class="fas fa-trash-alt me-1"></i>Löschen
+                    </button>
+                </form>
+            </div>
+        </div>
     </div>
 </div>
 {% endblock %}