feat: Add complete Verpachtung management system

- Add LandVerpachtung model with Land and Paechter relationships
- Implement full CRUD operations for Verpachtungen
- Add responsive Bootstrap templates with JavaScript calculations
- Integrate document linking functionality similar to other entities
- Add navigation links and URL patterns
- Include CSV import support for Paechter data
- Fix template encoding issues for proper UTF-8 support
- Enhance administration interface with Verpachtung CSV import

This implements the complete Verpachtung management feature requested,
allowing users to manage land lease agreements with proper relationships
to properties (Laenderei) and tenants (Paechter), following the same
patterns as Foerderung/Destinataer relationships.

app/templates/base.html

@@ -331,6 +331,13 @@
                                 <i class="fas fa-plus me-2"></i>Neue Länderei
                             </a></li>
                             <li><hr class="dropdown-divider"></li>
+                            <li><a class="dropdown-item" href="{% url 'stiftung:verpachtung_list' %}">
+                                <i class="fas fa-handshake me-2"></i>Alle Verpachtungen
+                            </a></li>
+                            <li><a class="dropdown-item" href="{% url 'stiftung:verpachtung_create' %}">
+                                <i class="fas fa-plus me-2"></i>Neue Verpachtung
+                            </a></li>
+                            <li><hr class="dropdown-divider"></li>
                             <li><a class="dropdown-item" href="{% url 'stiftung:land_abrechnung_list' %}">
                                 <i class="fas fa-calculator me-2"></i>Abrechnungen
                             </a></li>