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/entities/planned_leave.py

@@ -14,5 +14,13 @@ class PlannedLeaveDay:
     date: date
     day_type: str  # 'sjukpenning' | 'lagsta' | 'vacation'
     is_dubbeldag: bool = False  # True → counts against the 60-day dubbeldagar quota
+    # Which `LeaveWish` produced this day (nullable: ad-hoc/legacy/unattributable
+    # days have none). Lets the UI click a multi-day band on a calendar and edit
+    # it as one period instead of one day at a time.
+    wish_id: UUID | None = None
+    # True once the user hand-edits this day. A later task makes the allocator
+    # refuse to retype a pinned day on replan; this field only carries that
+    # state through the stack so far.
+    is_pinned: bool = False
     created_at: datetime | None = None
     updated_at: datetime | None = None