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/migrations/versions/0031_overtime_cap_expiry.py

@@ -0,0 +1,33 @@
+"""overtime cap and expiry config
+
+Revision ID: 0031
+Revises: 0030
+Create Date: 2026-05-25
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = '0031'
+down_revision = '0030'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Companies: Kappung + Verfall
+    op.add_column('companies', sa.Column('overtime_cap_hours', sa.Integer(), nullable=True))
+    op.add_column('companies', sa.Column('overtime_expiry_enabled', sa.Boolean(), nullable=False, server_default='false'))
+    op.add_column('companies', sa.Column('overtime_expiry_month', sa.Integer(), nullable=False, server_default='3'))
+    op.add_column('companies', sa.Column('overtime_expiry_day', sa.Integer(), nullable=False, server_default='31'))
+    op.add_column('companies', sa.Column('overtime_max_carryover_hours', sa.Integer(), nullable=True))
+    # OvertimeBalance: Verfall-Zeitstempel
+    op.add_column('overtime_balances', sa.Column('last_expiry_applied_at', sa.DateTime(timezone=True), nullable=True))
+
+
+def downgrade():
+    op.drop_column('companies', 'overtime_cap_hours')
+    op.drop_column('companies', 'overtime_expiry_enabled')
+    op.drop_column('companies', 'overtime_expiry_month')
+    op.drop_column('companies', 'overtime_expiry_day')
+    op.drop_column('companies', 'overtime_max_carryover_hours')
+    op.drop_column('overtime_balances', 'last_expiry_applied_at')