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/pages/Settings.test.tsx

@@ -117,7 +117,6 @@ describe('Settings', () => {
         workspace: {
           id: 'ws-1',
           name: 'Test Workspace',
-          planningGoal: 'balanced',
           planningMode: 'solo',
           createdAt: '2026-01-01',
         },
@@ -164,7 +163,6 @@ describe('Settings', () => {
     vi.mocked(api.updateWorkspace).mockResolvedValue({
       id: 'ws-1',
       name: 'Test Workspace',
-      planningGoal: 'balanced',
       planningMode: 'solo',
       createdAt: '2026-01-01',
     })
@@ -190,7 +188,6 @@ describe('Settings', () => {
         workspace: {
           id: 'ws-1',
           name: 'Test Workspace',
-          planningGoal: 'balanced',
           planningMode: 'solo',
           createdAt: '2026-01-01',
         },
@@ -283,14 +280,13 @@ describe('Settings', () => {
       expect(screen.getByTestId('planning-mode-solo')).toBeChecked()
     })
 
-    it('saves the selected planning mode together with the planning goal', async () => {
+    it('saves the selected planning mode', async () => {
       renderSettings()
       fireEvent.click(screen.getByTestId('planning-mode-couple'))
       fireEvent.click(screen.getByTestId('save-planning-goal-button'))
 
       await waitFor(() =>
         expect(api.updateWorkspace).toHaveBeenCalledWith('ws-1', {
-          planningGoal: 'balanced',
           planningMode: 'couple',
         }),
       )