feat: Überstunden-Kappung + Jahresverfall pro Firma konfigurierbar

Backend:
- Company: overtime_cap_hours, overtime_expiry_enabled/month/day,
  overtime_max_carryover_hours
- OvertimeBalance: last_expiry_applied_at
- Migration 0031: neue Spalten in companies + overtime_balances
- _recalculate_overtime_balance: Kappung direkt nach Berechnung
- apply_overtime_expiry_if_needed(): lazy Verfall beim Balance-Abruf
- GET /absences/overtime-balance: prüft + wendet Verfall automatisch an
- POST /absences/overtime-balance/apply-expiry: manueller Trigger (Admin)

Frontend:
- CompanySettingsPage: neuer Block 'Überstunden-Konto'
  - Toggle Kappungsgrenze + Stunden-Input
  - Toggle Jahresverfall + Stichtag (Tag/Monat) + max. Übertrag
  - 'Verfall anwenden'-Button für Admins

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/overtime_balance.py

@@ -34,6 +34,7 @@ class OvertimeBalance(Base):
     total_hours: Mapped[Decimal] = mapped_column(Numeric(8, 2), default=Decimal("0"))
     taken_hours: Mapped[Decimal] = mapped_column(Numeric(8, 2), default=Decimal("0"))
     last_calculated: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    last_expiry_applied_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()