Let a household choose to be paid for the red days

Asking for a full week off never spent anything on the Saturday, the
Sunday or a holiday, and a test now says so outright rather than leaving
it to be rediscovered. Those days are free because vacation cannot be
taken on a day that is not a working day.

Parental benefit is a payment rather than a schedule, though, so it can
be drawn on a red day to turn an unpaid day off into a paid one — at the
cost of a day the family would otherwise keep. That is now a workspace
setting, off by default, and it never reaches for a vacation day. With it
off the planner behaves exactly as before.

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

backend/app/entities/workspace.py

@@ -10,3 +10,9 @@ class Workspace:
     id: UUID
     name: str
     created_at: datetime | None = None
+    # §todo item 2 — per-workspace opt-in, default False: allow the planning
+    # engine to fund a free day (Saturday, Sunday, public holiday) with a
+    # parental-benefit day instead of leaving it unpaid. Never vacation (see
+    # `app.services.allocation_service.REASON_FP_RED_DAY`). Threaded into the
+    # allocator by `AllocationInputLoader.load` as `AllocationInput.take_pl_on_red_days`.
+    take_pl_on_red_days: bool = False