Link planned days to their wish, fix the age-4 cap, drop the planning goal

Three changes that together unblock treating a calendar period as one
editable object.

Migration 008 adds `planned_leave_day.wish_id` (nullable, ON DELETE SET NULL)
so a persisted day knows which `leave_wish` produced it, `is_pinned` to mark a
day the user hand-edited, and `leave_wish.target_days` so a period drawn on a
calendar need not be a whole number of weeks. Apply-time identity still keys on
day_type/is_dubbeldag only, so a wish_id-only difference is a metadata update
rather than a collision.

The age-4 savable ceiling was applying the per-child figure of 96 to each
parent's pool, modelling a two-parent family as able to save 192 days past the
child's fourth birthday. `resolve_age4_savable_cap` is now the single place
that decides 48 (couple with two parents) versus 96 (everyone else), and both
AnalyticsService and the allocator read it.

Migration 009 drops `workspace.planning_goal` and `leave_wish.goal_override`.
The allocator's tier-3 policy collapses to one rule; `salary_type` still
branches, since that reflects what a vacation day actually costs a parent
rather than a user preference.

backend/app/routers/leave_wishes.py

@@ -1,8 +1,8 @@
 """HTTP routes for leave wishes.
 
 Routers stay thin: validate via Pydantic, call the service, map domain errors
-to HTTP. Business logic (recurrence/parent_mode/goal_override validation,
-month-day range checks) lives in LeaveWishService. See PLANNING_ENGINE_SPEC.md §4.
+to HTTP. Business logic (recurrence/parent_mode validation, month-day range
+checks) lives in LeaveWishService. See PLANNING_ENGINE_SPEC.md §4.
 """
 
 from uuid import UUID
@@ -35,13 +35,13 @@ async def create_leave_wish(
             window_end_month=payload.window_end_month,
             window_end_day=payload.window_end_day,
             target_weeks=payload.target_weeks,
+            target_days=payload.target_days,
             recurrence=payload.recurrence,
             first_year=payload.first_year,
             last_year=payload.last_year,
             priority=payload.priority,
             parent_mode=payload.parent_mode,
             parent_personal_data_uuid=payload.parent_personal_data_uuid,
-            goal_override=payload.goal_override,
             window_mode=payload.window_mode,
         )
     except ValidationError as exc:
@@ -77,13 +77,13 @@ async def update_leave_wish(
             window_end_month=payload.window_end_month,
             window_end_day=payload.window_end_day,
             target_weeks=payload.target_weeks,
+            target_days=payload.target_days,
             recurrence=payload.recurrence,
             first_year=payload.first_year,
             last_year=payload.last_year,
             priority=payload.priority,
             parent_mode=payload.parent_mode,
             parent_personal_data_uuid=payload.parent_personal_data_uuid,
-            goal_override=payload.goal_override,
             window_mode=payload.window_mode,
             _clear_child_id="child_id" in fields_set and payload.child_id is None,
             _clear_last_year="last_year" in fields_set and payload.last_year is None,
@@ -91,10 +91,10 @@ async def update_leave_wish(
                 "parent_personal_data_uuid" in fields_set
                 and payload.parent_personal_data_uuid is None
             ),
-            _clear_goal_override=("goal_override" in fields_set and payload.goal_override is None),
             _clear_name_message_key=(
                 "name_message_key" in fields_set and payload.name_message_key is None
             ),
+            _clear_target_days="target_days" in fields_set and payload.target_days is None,
         )
     except ValidationError as exc:
         raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, str(exc)) from exc