feat: Implement quarterly confirmation system with automatic support payments

- Add VierteljahresNachweis model for quarterly document tracking
- Remove studiennachweis_erforderlich field (now always required)
- Fix modal edit view to include studiennachweis section
- Implement automatic DestinataerUnterstuetzung creation when requirements met
- Set payment due dates to exact quarter end dates (Mar 31, Jun 30, Sep 30, Dec 31)
- Add quarterly confirmation CRUD views with modal and full-screen editing
- Update templates with comprehensive quarterly management interface
- Include proper validation, status tracking, and progress indicators

app/stiftung/urls.py

@@ -348,4 +348,20 @@ urlpatterns = [
     # Gramps integration (probe)
     path("api/gramps/search/", views.gramps_search_api, name="gramps_search_api"),
     path("api/gramps/debug/", views.gramps_debug_api, name="gramps_debug_api"),
+    # Quarterly Confirmations
+    path(
+        "quarterly-confirmations/create/<uuid:destinataer_id>/",
+        views.quarterly_confirmation_create,
+        name="quarterly_confirmation_create",
+    ),
+    path(
+        "quarterly-confirmations/<uuid:pk>/edit/",
+        views.quarterly_confirmation_edit,
+        name="quarterly_confirmation_edit",
+    ),
+    path(
+        "quarterly-confirmations/<uuid:pk>/update/",
+        views.quarterly_confirmation_update,
+        name="quarterly_confirmation_update",
+    ),
 ]