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.

frontend/src/i18n/en/plan.ts

@@ -104,13 +104,6 @@ export const plan = {
   'planResults.wished': 'wished',
   'planResults.yearHeader': 'Year',
   'planResults.yearOverviewTitle': 'Overview by year',
-  'planningGoalSelector.balanced': 'Balanced',
-  'planningGoalSelector.balancedHelp': 'A trade-off between time at home and income.',
-  'planningGoalSelector.legend': 'Planning goal',
-  'planningGoalSelector.maxMoney': 'Most money',
-  'planningGoalSelector.maxMoneyHelp': 'Protect your income, e.g. by placing vacation around weekends and public holidays.',
-  'planningGoalSelector.maxTime': 'Most time at home',
-  'planningGoalSelector.maxTimeHelp': 'Save days and extend your total leave time, with lower income now.',
   'planningModeSelector.couple': 'Me and my partner',
   'planningModeSelector.legend': 'Are you planning alone or together with a partner?',
   'planningModeSelector.solo': 'Just me',