fix: resolve Python code quality issues

- Fix undefined variable 'e' in PDF generator error handling
- Replace undefined 'Verpachtung' model references with 'LandVerpachtung'
- Fix all import and object access references to use correct model name
- Resolve all flake8 F821 undefined name errors

This addresses all code quality failures that were blocking CI pipeline.

app/stiftung/models.py

@@ -1128,8 +1128,8 @@ class DokumentLink(models.Model):
         """Gibt die verknüpfte Verpachtung zurück"""
         if self.verpachtung_id:
             try:
-                return Verpachtung.objects.get(pk=self.verpachtung_id)
-            except Verpachtung.DoesNotExist:
+                return LandVerpachtung.objects.get(pk=self.verpachtung_id)
+            except LandVerpachtung.DoesNotExist:
                 return None
         return None