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/scripts/demo_plan.py

@@ -123,7 +123,6 @@ def _build_input() -> tuple[AllocationInput, dict]:
     alloc_input = AllocationInput(
         today=TODAY,
         horizon_end=horizon_end,
-        planning_goal="balanced",
         planning_mode="couple",
         children=[child],
         parents=[parent_1, parent_2],

backend/scripts/seed_demo.py

@@ -118,7 +118,7 @@ def _build_services(conn: aiosqlite.Connection) -> DemoServices:
     parental_leave = SQLiteParentalLeaveRepository(conn)
     planned_leave = SQLitePlannedLeaveRepository(conn)
 
-    analytics = AnalyticsService(parental_leave, planned_leave, children, personal_data)
+    analytics = AnalyticsService(parental_leave, planned_leave, children, personal_data, workspaces)
 
     return DemoServices(
         auth=AuthService(users, personal_data, workspaces),
@@ -214,9 +214,7 @@ async def _seed(conn: aiosqlite.Connection, *, reset: bool) -> None:
         email=DEMO_EMAIL, password=DEMO_PASSWORD, display_name=DEMO_NAME
     )
     workspace_id = alex.workspace_id
-    await services.workspace.update_workspace(
-        workspace_id, planning_mode="couple", planning_goal="balanced"
-    )
+    await services.workspace.update_workspace(workspace_id, planning_mode="couple")
 
     # -- 2. Partner (no login — shared/partner planning needs a second parent
     # on the workspace, but nothing requires them to have registered) -------