feat: added ticket history

ticketsystem/templates/ticketsystem/detail.html

@@ -132,5 +132,22 @@
         <button type="submit">Absenden</button>
     </form>
     {% endif %}
+    <div style="margin-top: 3rem;">
+  <h2>🕓 Bearbeitungshistorie</h2>
+  {% if ticket.history.exists %}
+    <ul style="list-style: none; padding: 0;">
+      {% for entry in ticket.history.all %}
+        <li style="margin-bottom: 1rem; background-color: #f9f9f9; border-left: 4px solid #ccc; padding: 0.5rem 1rem;">
+          <strong>{{ entry.changed_by.username }}</strong>
+          hat <code>{{ entry.field }}</code> geändert:<br>
+          <em>{{ entry.old_value }}</em> → <strong>{{ entry.new_value }}</strong><br>
+          <small>am {{ entry.changed_at|date:"d.m.Y H:i" }}</small>
+        </li>
+      {% endfor %}
+    </ul>
+  {% else %}
+    <p><em>Keine Änderungen bisher.</em></p>
+  {% endif %}
+</div>
 </div>
 {% endblock %}