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/services/workspace_service.py

@@ -75,17 +75,11 @@ class WorkspaceService:
         self,
         workspace_id: UUID,
         name: str | None = None,
-        planning_goal: str | None = None,
         planning_mode: str | None = None,
     ) -> Workspace:
         ws = await self.get_workspace(workspace_id)
         if name is not None:
             ws.name = name
-        if planning_goal is not None:
-            valid = {"max_time", "max_money", "balanced"}
-            if planning_goal not in valid:
-                raise ValidationError(f"planning_goal must be one of {valid}")
-            ws.planning_goal = planning_goal
         if planning_mode is not None:
             valid_modes = {"solo", "couple"}
             if planning_mode not in valid_modes: