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/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(