Delete date helpers nothing calls

A hand-rolled month-arithmetic module survived the move to dateutil with no
caller left but its own tests, and two predicates alongside it were never
wired up at all. An error import was also being deferred inside a function
for a circular dependency that does not exist.

backend/app/services/analytics_service.py

@@ -29,7 +29,7 @@ from app.repositories.parental_leave_repository import ParentalLeaveRepository
 from app.repositories.personal_data_repository import PersonalDataRepository
 from app.repositories.planned_leave_repository import PlannedLeaveRepository
 from app.repositories.workspace_repository import WorkspaceRepository
-from app.utils.errors import NotFoundError
+from app.utils.errors import DubbeldagError, NotFoundError
 from app.utils.tip_predicates import days_left_at_age_4_projection, resolve_age4_savable_cap
 
 
@@ -188,8 +188,6 @@ class AnalyticsService:
 
         Called from the planning service before persisting a new day.
         """
-        from app.utils.errors import DubbeldagError
-
         child = await self._children.get_by_id(child_id)
         if child is None:
             raise NotFoundError(f"Child {child_id} not found")