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/tests/unit/test_plan_router.py

@@ -216,6 +216,10 @@ class TestProposeAge4ProjectionsOverTheWire:
         assert isinstance(projection["projected_left_at_b4"], int)
         assert isinstance(projection["savable_after_b4"], int)
         assert isinstance(projection["expiring_at_b4"], int)
+        # savable_cap_at_b4 (the resolved 96-per-child vs. 48-per-parent
+        # ceiling) is exactly the kind of field this test class exists to
+        # catch going missing at the API boundary.
+        assert projection["savable_cap_at_b4"] == 96  # solo workspace, one parent
         # No leave planned at all before B4 in this scenario -> everything
         # above the savable ceiling is forecast to expire, matching the
         # surplus-advice test above.