fix: Correct LandVerpachtung admin field references

- Fix admin list_display and list_filter to use actual model fields
- Update field names: pachtpreis_qm -> pachtzins_pauschal, laufzeit_von -> pachtbeginn, etc.
- Add comprehensive fieldsets matching actual LandVerpachtung model structure
- Resolves SystemCheckError preventing migrations in production

app/stiftung/admin.py

@@ -309,29 +309,40 @@ class LandVerpachtungAdmin(admin.ModelAdmin):
     list_display = [
         "land",
         "paechter",
-        "pachtpreis_qm",
-        "laufzeit_von",
-        "laufzeit_bis",
+        "pachtzins_pauschal",
+        "pachtbeginn",
+        "pachtende",
         "status_display",
         "erstellt_am",
     ]
-    list_filter = ["status", "laufzeit_von", "laufzeit_bis", "erstellt_am"]
-    search_fields = ["land__lfd_nr", "land__gemeinde", "paechter__vorname", "paechter__nachname"]
+    list_filter = ["status", "pachtbeginn", "pachtende", "erstellt_am"]
+    search_fields = ["land__lfd_nr", "land__gemeinde", "paechter__vorname", "paechter__nachname", "vertragsnummer"]
     ordering = ["-erstellt_am"]
     readonly_fields = ["id", "erstellt_am", "aktualisiert_am"]
 
     fieldsets = (
         ("Verpachtungsdetails", {
-            "fields": ("land", "paechter", "pachtpreis_qm", "status")
+            "fields": ("land", "paechter", "vertragsnummer", "status")
         }),
         ("Laufzeit", {
-            "fields": ("laufzeit_von", "laufzeit_bis", "kuendigungsfrist_monate")
+            "fields": ("pachtbeginn", "pachtende", "verlaengerung_klausel")
         }),
-        ("Finanziell", {
-            "fields": ("kaution", "indexierung", "zahlungsintervall")
+        ("Fläche", {
+            "fields": ("verpachtete_flaeche",)
+        }),
+        ("Pachtzins", {
+            "fields": ("pachtzins_pauschal", "pachtzins_pro_ha", "zahlungsweise")
+        }),
+        ("Umsatzsteuer", {
+            "fields": ("ust_option", "ust_satz"),
+            "classes": ("collapse",)
+        }),
+        ("Umlagen", {
+            "fields": ("grundsteuer_umlage", "versicherungen_umlage", "verbandsbeitraege_umlage", "jagdpacht_anteil_umlage"),
+            "classes": ("collapse",)
         }),
         ("Zusatzinformationen", {
-            "fields": ("notizen", "besondere_vereinbarungen"),
+            "fields": ("bemerkungen",),
             "classes": ("collapse",)
         }),
         ("System", {