Add ALKIS Flurstückskennzeichen field for direct cadastre links

When an ALKIS identifier is set on a Land record, the button links to
ogc-api.nrw.de detail view instead of the imprecise TIM-Online search.
Falls back to TIM-Online when no ALKIS number is present.

Closes STI-57

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

app/templates/stiftung/land_list.html

@@ -249,10 +249,17 @@
                                            class="btn btn-sm btn-outline-primary" title="Anzeigen">
                                             <i class="fas fa-eye"></i>
                                         </a>
+                                        {% if land.alkis_kennzeichen %}
+                                        <a href="https://ogc-api.nrw.de/lika/v1/collections/flurstueck/Items/{{ land.alkis_kennzeichen|urlencode }}"
+                                           class="btn btn-sm btn-outline-success" title="Kataster" target="_blank" rel="noopener">
+                                            <i class="fas fa-map-marked-alt"></i>
+                                        </a>
+                                        {% else %}
                                         <a href="https://www.tim-online.nrw.de/tim-online2/?WFS_gemarkung={{ land.gemarkung|urlencode }}&WFS_flur={{ land.flur|urlencode }}&WFS_flurstueck={{ land.flurstueck|urlencode }}"
                                            class="btn btn-sm btn-outline-success" title="TIM-Online NRW" target="_blank" rel="noopener">
                                             <i class="fas fa-map-marked-alt"></i>
                                         </a>
+                                        {% endif %}
                                         <a href="{% url 'stiftung:land_update' land.pk %}"
                                            class="btn btn-sm btn-outline-warning" title="Bearbeiten">
                                             <i class="fas fa-edit"></i>