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/services/report_service.py

@@ -242,9 +242,63 @@ async def _recalculate_overtime_balance(
 
     bal.total_hours = Decimal(str(round(overtime, 2)))
     bal.last_calculated = datetime.utcnow()
+
+    # Kappung anwenden
+    company = await db.get(Company, user.company_id)
+    if company and company.overtime_cap_hours is not None:
+        cap = Decimal(str(company.overtime_cap_hours))
+        if bal.total_hours > cap:
+            bal.total_hours = cap
+
     return bal
 
 
+async def apply_overtime_expiry_if_needed(
+    bal: OvertimeBalance,
+    company,  # Company model
+    db: AsyncSession,
+) -> bool:
+    """
+    Prüft ob der Überstunden-Verfall angewendet werden muss und tut es ggf.
+    Gibt True zurück wenn Verfall angewendet wurde.
+    """
+    if not company or not company.overtime_expiry_enabled:
+        return False
+
+    today = date.today()
+    try:
+        expiry_this_year = date(today.year, company.overtime_expiry_month, company.overtime_expiry_day)
+    except ValueError:
+        # Ungültiges Datum (z.B. 31. Februar) – überspringen
+        return False
+
+    expiry_last_year_year = today.year - 1
+    try:
+        expiry_last_year = date(expiry_last_year_year, company.overtime_expiry_month, company.overtime_expiry_day)
+    except ValueError:
+        expiry_last_year = None
+
+    last_applicable_expiry = expiry_this_year if today >= expiry_this_year else expiry_last_year
+    if last_applicable_expiry is None:
+        return False
+
+    # Schon angewendet?
+    if bal.last_expiry_applied_at:
+        applied_date = bal.last_expiry_applied_at.date() if hasattr(bal.last_expiry_applied_at, 'date') else bal.last_expiry_applied_at
+        if applied_date >= last_applicable_expiry:
+            return False
+
+    # Verfall anwenden: available_hours auf max_carryover kappen
+    available = bal.total_hours - bal.taken_hours
+    if company.overtime_max_carryover_hours is not None:
+        max_carry = Decimal(str(company.overtime_max_carryover_hours))
+        if available > max_carry:
+            bal.total_hours = bal.taken_hours + max_carry
+
+    bal.last_expiry_applied_at = datetime.utcnow()
+    return True
+
+
 def _check_arbzg_day(entry: TimeEntry) -> list[str]:
     """ArbZG-Prüfung für einen einzelnen Zeiteintrag."""
     if entry.end_time is None: