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/schemas/company.py

@@ -24,6 +24,11 @@ class CompanyOut(BaseModel):
     mobile_stamping_enabled: bool = True
     overtime_overdraft_allowed: bool = True
     overtime_warning_threshold_hours: int = 0
+    overtime_cap_hours: int | None = None
+    overtime_expiry_enabled: bool = False
+    overtime_expiry_month: int = 3
+    overtime_expiry_day: int = 31
+    overtime_max_carryover_hours: int | None = None
     kiosk_require_approval: bool = True
     kiosk_track_current_user: bool = True
     kiosk_heartbeat_interval_sec: int = 30
@@ -39,6 +44,11 @@ class CompanyUpdate(BaseModel):
     mobile_stamping_enabled: bool | None = None
     overtime_overdraft_allowed: bool | None = None
     overtime_warning_threshold_hours: int | None = Field(None, ge=0)
+    overtime_cap_hours: int | None = Field(None, ge=1, le=9999)
+    overtime_expiry_enabled: bool | None = None
+    overtime_expiry_month: int | None = Field(None, ge=1, le=12)
+    overtime_expiry_day: int | None = Field(None, ge=1, le=31)
+    overtime_max_carryover_hours: int | None = Field(None, ge=0, le=9999)
     kiosk_require_approval: bool | None = None
     kiosk_track_current_user: bool | None = None
     kiosk_heartbeat_interval_sec: int | None = Field(None, ge=10, le=120)