fix(tasks): add queryset stub to satisfy drf-spectacular schema generation

TasksViewSet.get_queryset() accesses request.user, which drf-spectacular
cannot provide during static schema generation.  Adding a class-level
queryset = PaperlessTask.objects.none() gives spectacular a model to
introspect without invoking get_queryset(), eliminating both warnings
and the test_valid_schema failure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/documents/views.py

@@ -3786,6 +3786,8 @@ class TasksViewSet(ReadOnlyModelViewSet[PaperlessTask]):
         "wait_time_seconds",
     ]
     ordering = ["-date_created"]
+    # Needed for drf-spectacular schema generation (get_queryset touches request.user)
+    queryset = PaperlessTask.objects.none()
 
     # v9 backwards compat: maps old "type" query param values to new TriggerSource
     _V9_TYPE_TO_TRIGGER_SOURCE = {