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/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'),
+        ),
+    ]