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/models/leave_wish.py

@@ -30,6 +30,9 @@ class LeaveWishCreate(BaseModel):
     window_end_day: int = Field(ge=1, le=31)
 
     target_weeks: int = Field(ge=1, le=52)
+    # A calendar-drawn period is not a whole number of weeks. None (default)
+    # falls back to `target_weeks * 7` — see `LeaveWish.target_span_days()`.
+    target_days: int | None = Field(default=None, ge=1)
     recurrence: str = "once"  # 'yearly' | 'once'
     first_year: int = Field(ge=2000, le=2200)
     last_year: int | None = Field(default=None, ge=2000, le=2200)
@@ -38,8 +41,6 @@ class LeaveWishCreate(BaseModel):
     parent_mode: str = "either"  # 'either' | 'both' | 'specific'
     parent_personal_data_uuid: UUID | None = None
 
-    goal_override: str | None = None  # 'max_time' | 'max_money' | 'balanced' | None
-
     # PLANNING_ENGINE_SPEC.md §10.1 — 'fixed': window_start_*/window_end_* are the
     # exact placement (default). 'flexible': the same fields describe the
     # bounding range the allocator searches within for the best-costing window.
@@ -63,6 +64,10 @@ class LeaveWishUpdate(BaseModel):
     window_end_day: int | None = Field(None, ge=1, le=31)
 
     target_weeks: int | None = Field(None, ge=1, le=52)
+    # Present + null means "clear it" (fall back to target_weeks); absent
+    # means "leave unchanged" — same convention as the other nullable fields
+    # on this model (see `_clear_*` handling in `LeaveWishService.update`).
+    target_days: int | None = Field(default=None, ge=1)
     recurrence: str | None = None
     first_year: int | None = Field(None, ge=2000, le=2200)
     last_year: int | None = Field(None, ge=2000, le=2200)
@@ -71,8 +76,6 @@ class LeaveWishUpdate(BaseModel):
     parent_mode: str | None = None
     parent_personal_data_uuid: UUID | None = None
 
-    goal_override: str | None = None
-
     window_mode: str | None = None  # 'fixed' | 'flexible'
 
 
@@ -89,6 +92,7 @@ class LeaveWishResponse(BaseModel):
     window_end_day: int
 
     target_weeks: int
+    target_days: int | None
     recurrence: str
     first_year: int
     last_year: int | None
@@ -97,8 +101,6 @@ class LeaveWishResponse(BaseModel):
     parent_mode: str
     parent_personal_data_uuid: UUID | None
 
-    goal_override: str | None
-
     window_mode: str
 
     created_at: datetime | None
@@ -117,13 +119,13 @@ class LeaveWishResponse(BaseModel):
             window_end_month=entity.window_end_month,
             window_end_day=entity.window_end_day,
             target_weeks=entity.target_weeks,
+            target_days=entity.target_days,
             recurrence=entity.recurrence,
             first_year=entity.first_year,
             last_year=entity.last_year,
             priority=entity.priority,
             parent_mode=entity.parent_mode,
             parent_personal_data_uuid=entity.parent_personal_data_uuid,
-            goal_override=entity.goal_override,
             window_mode=entity.window_mode,
             created_at=entity.created_at,
             updated_at=entity.updated_at,