fix: 8 pre-existing Test-Fehler behoben

1. Krankmeldungen an Wochenenden erlaubt (working_days=0 für SICK)
   Medizinisch korrekt: Krankmeldungen können auch Wochenendtage umfassen.
   Behebt: test_create_absence_sick_auto_approved, test_quick_sick_*,
           test_pull_includes_today_absence_with_category, test_sick_stats_*

2. Self-Approval-Schutz in Tests berücksichtigt
   abs_approver_headers / time_approver_headers: zweiter Admin je Company.
   Behebt: test_approve_absence, test_balance_deducted_after_approve, test_approve_entry

3. test_export_invalid_format: "pdf" → "xml" (pdf ist jetzt valides Format)

Ergebnis: 134/134 passed.

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

backend/tests/test_reports.py

@@ -269,7 +269,7 @@ async def test_export_invalid_format(client: AsyncClient, report_headers):
     today = date.today()
     resp = await client.get(
         "/api/v1/reports/time/export",
-        params={"date_from": str(today.replace(day=1)), "date_to": str(today), "format": "pdf"},
+        params={"date_from": str(today.replace(day=1)), "date_to": str(today), "format": "xml"},
         headers=report_headers,
     )
     assert resp.status_code == 422