Give the analytics overview a per-parent, per-year balance series

Each parent projection now carries one entry per calendar year from
birth to the fourth birthday: vacation earned, carried in, planned,
left, carried out and expiring under Semesterlagen's 20-day floor and
5-year carry-over, plus parental-leave days used and left at year end
with the age-4 clamp applied in the birthday year. The overview sums
them into per-year totals. Frontend types, mapper and fixtures follow.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XBRLomdLK9jF7wU7a8AA6H

backend/app/config.py

@@ -62,6 +62,12 @@ class LeaveRules(BaseModel):
     # allocator actually enforces (PLANNING_ENGINE_SPEC.md §9.2).
     min_vacation_days_per_year: int = 20
 
+    # Semesterlagen: a vacation day saved above the floor (see
+    # `min_vacation_days_per_year`) may be carried forward for this many
+    # years before it expires. Used by `AnalyticsService`'s per-year vacation
+    # carry-forward bank (§ per-parent-per-year balance series).
+    vacation_carry_forward_years: int = 5
+
     @property
     def total_days_per_child(self) -> int:
         return self.sjukpenning_days + self.lagsta_niva_days