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/planned_leave.py

@@ -19,6 +19,9 @@ class PlannedLeaveDayCreate(BaseModel):
     date: date
     day_type: str = Field(pattern="^(sjukpenning|lagsta|vacation)$")
     is_dubbeldag: bool = False
+    # Which `LeaveWish` produced this day; None = ad-hoc.
+    wish_id: UUID | None = None
+    is_pinned: bool = False
 
 
 class PlannedLeaveDayUpdate(BaseModel):
@@ -26,6 +29,8 @@ class PlannedLeaveDayUpdate(BaseModel):
 
     day_type: str | None = Field(None, pattern="^(sjukpenning|lagsta|vacation)$")
     is_dubbeldag: bool | None = None
+    wish_id: UUID | None = None
+    is_pinned: bool | None = None
 
 
 class PlannedLeaveDayResponse(BaseModel):
@@ -36,6 +41,8 @@ class PlannedLeaveDayResponse(BaseModel):
     date: date
     day_type: str
     is_dubbeldag: bool
+    wish_id: UUID | None
+    is_pinned: bool
     created_at: datetime | None
     updated_at: datetime | None
 
@@ -49,6 +56,8 @@ class PlannedLeaveDayResponse(BaseModel):
             date=entity.date,
             day_type=entity.day_type,
             is_dubbeldag=entity.is_dubbeldag,
+            wish_id=entity.wish_id,
+            is_pinned=entity.is_pinned,
             created_at=entity.created_at,
             updated_at=entity.updated_at,
         )