Let a day removed from a period stay removed

Taking one day out of a period the planner placed did nothing that
lasted: the period still asked for a full week, so the next sync put the
day straight back. The option was disabled rather than lying about it.

A removed day is now recorded as its own thing — no leave for this
parent, this child, this date — and the planner honours it on every
later sync. It is kept apart from the working-day overrides, which
answer a different question about the calendar rather than about the
plan.

The day belongs to the parent whose band was clicked, so removing one
parent's day leaves their partner's alone; where both parents were to be
off on one funding arrangement, the partner is simply funded on their
own instead. A removed day shortens what the period is asking for rather
than counting as a day the plan failed to find, so the advice about
closing a shortfall does not start arguing for a day nobody wants.

Nothing is removed without a way back: the toast undoes it, and the day
itself is marked in the calendar and offers to restore.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ei7gCZzdd751uZtPzb1Pep

backend/app/repositories/planned_day_exclusion_repository.py

@@ -0,0 +1,28 @@
+"""Abstract repository interface for PlannedDayExclusion.
+
+Adds entity-specific queries on top of the generic CRUD contract. Concrete
+implementations live in app/repositories_impl/<backend>/.
+"""
+
+from abc import abstractmethod
+from datetime import date
+from uuid import UUID
+
+from app.entities.planned_day_exclusion import PlannedDayExclusion
+from app.repositories.base import Repository
+
+
+class PlannedDayExclusionRepository(Repository[PlannedDayExclusion]):
+    @abstractmethod
+    async def get_by_natural_key(
+        self,
+        workspace_id: UUID,
+        child_id: UUID,
+        parent_personal_data_uuid: UUID,
+        day: date,
+    ) -> PlannedDayExclusion | None:
+        """Return the entry for a given (workspace, child, parent, date) (UNIQUE), or None."""
+
+    @abstractmethod
+    async def list_by_workspace(self, workspace_id: UUID) -> list[PlannedDayExclusion]:
+        """Return all exclusions for a workspace, ordered by date."""