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/stiftung/forms/land.py

@@ -21,6 +21,7 @@ class LandForm(forms.ModelForm):
             "flur",
             "flurstueck",
             "adresse",
+            "alkis_kennzeichen",
             # Flächenangaben
             "groesse_qm",
             "gruenland_qm",

app/stiftung/migrations/0054_add_alkis_kennzeichen.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2026-03-13 21:57
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0053_geschichte_dokumente_m2m'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='land',
+            name='alkis_kennzeichen',
+            field=models.CharField(blank=True, help_text='z.B. 05300800400030______ — für direkte Verlinkung zum Katasteramt', max_length=30, null=True, verbose_name='ALKIS Flurstückskennzeichen'),
+        ),
+    ]

app/stiftung/models/land.py

@@ -137,6 +137,13 @@ class Land(models.Model):
     adresse = models.CharField(
         max_length=200, null=True, blank=True, verbose_name="Adresse/Ortsangabe"
     )
+    alkis_kennzeichen = models.CharField(
+        max_length=30,
+        null=True,
+        blank=True,
+        verbose_name="ALKIS Flurstückskennzeichen",
+        help_text="z.B. 05300800400030______ — für direkte Verlinkung zum Katasteramt",
+    )
 
     # Flächenangaben
     groesse_qm = models.DecimalField(

app/templates/stiftung/land_detail.html

@@ -25,10 +25,17 @@
         </div>
         <div class="col-md-4 text-end">
             <div class="btn-group" role="group">
+                {% if land.alkis_kennzeichen %}
+                <a href="https://ogc-api.nrw.de/lika/v1/collections/flurstueck/Items/{{ land.alkis_kennzeichen|urlencode }}"
+                   class="btn btn-outline-success" title="Flurstück im Kataster anzeigen" target="_blank" rel="noopener">
+                    <i class="fas fa-map-marked-alt me-2"></i>Kataster
+                </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-outline-success" title="TIM-Online NRW öffnen" target="_blank" rel="noopener">
                     <i class="fas fa-map-marked-alt me-2"></i>TIM-Online
                 </a>
+                {% endif %}
                 <a href="{% url 'stiftung:land_update' land.pk %}" class="btn btn-warning">
                     <i class="fas fa-edit me-2"></i>Bearbeiten
                 </a>

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>