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/models/workspace.py

@@ -15,14 +15,12 @@ class WorkspaceCreate(BaseModel):
 
 class WorkspaceUpdate(BaseModel):
     name: str | None = Field(default=None, min_length=1, max_length=100)
-    planning_goal: str | None = None
     planning_mode: str | None = None
 
 
 class WorkspaceResponse(BaseModel):
     id: UUID
     name: str
-    planning_goal: str
     planning_mode: str
     created_at: datetime | None
 
@@ -31,7 +29,6 @@ class WorkspaceResponse(BaseModel):
         return cls(
             id=ws.id,
             name=ws.name,
-            planning_goal=ws.planning_goal,
             planning_mode=ws.planning_mode,
             created_at=ws.created_at,
         )