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/leave_wish.py

@@ -41,10 +41,6 @@ class LeaveWish:
     parent_mode: str  # 'either' | 'both' | 'specific'
     parent_personal_data_uuid: UUID | None  # required when parent_mode == 'specific'
 
-    # Per-window override of workspace planning_goal. Nullable, no UI in v1 —
-    # the field exists now so enabling it later needs no migration.
-    goal_override: str | None = None  # 'max_time' | 'max_money' | 'balanced' | None
-
     # PLANNING_ENGINE_SPEC.md §10.1 — 'fixed' (default): window_start_*/window_end_*
     # are the exact placement, as above. 'flexible': the SAME four fields instead
     # describe the BOUNDING RANGE the allocator searches inside for the
@@ -65,9 +61,19 @@ class LeaveWish:
     # revert to one.
     name_message_key: str | None = None
 
+    # A calendar-drawn period (e.g. dragging a band on the calendar UI) is not
+    # a whole number of weeks. NULL (every row created before this field
+    # existed, and any wish still specified in weeks) falls back to
+    # `target_weeks * 7` — see `target_span_days()`.
+    target_days: int | None = None
+
     created_at: datetime | None = None
     updated_at: datetime | None = None
 
+    def target_span_days(self) -> int:
+        """Wanted duration in days. `target_days` wins when set; otherwise weeks * 7."""
+        return self.target_days if self.target_days is not None else self.target_weeks * 7
+
     def window_wraps(self) -> bool:
         """Whether (end_month, end_day) sorts before (start_month, start_day).