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",