feat: agent-11 PR1 – Vertretung, Storno-Re-Genehmigung, Kommentare

Abwesenheits-Modul abgerundet (Feature-Parität mit Urlaubsverwaltung):

- Vertretung: Overlap-Warnung beim Anlegen, E-Mail an Vertretung bei
  Genehmigung, GET /absences/?as_substitute=true, neuer schlanker
  GET /users/colleagues (alle Rollen, RLS-gefenced) für die Auswahl;
  Vertreter-Dropdown + Anzeige in der Liste.
- Stornierung mit Re-Genehmigung: neuer Status CANCELLATION_REQUESTED,
  POST /absences/{id}/request-cancellation; Manager genehmigt/lehnt über
  bestehende approve/reject ab (Urlaub + FZA-Rückbuchung via _apply_cancellation).
- Kommentare: Model AbsenceComment (company_id-RLS), GET/POST comments,
  System-Kommentare bei Statuswechsel, AbsenceCommentsModal.
- Fix: CalDAV fire-and-forget nutzte die Request-Session weiter (in Tests
  geteilt -> "another operation in progress"); jetzt sync_*_bg mit eigener
  Session + RLS-Bypass.

Migration 0035. 178/178 Tests grün. Deployed auf 137 + 164.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/routers/absences.py

@@ -12,12 +12,15 @@ from app.schemas.absence import (
     AbsenceCreate,
     AbsenceListResponse,
     AbsenceOut,
+    AbsenceCommentCreate,
+    AbsenceCommentOut,
     AbsenceReject,
     AbsenceUpdate,
     AbsenceTypeCreate,
     AbsenceTypeOut,
     AbsenceTypeUpdate,
     CalendarEntry,
+    CancellationRequest,
     CertificateMarkIn,
     OvertimeBalanceOut,
     PublicHolidayCreate,
@@ -256,11 +259,13 @@ async def list_absences(
     type_id: UUID | None = Query(None),
     status: AbsenceStatus | None = Query(None),
     year: int | None = Query(None),
+    as_substitute: bool = Query(False),
     db: AsyncSession = Depends(get_db),
 ):
     total, absences = await absence_service.list_absences(
         current_user.company_id, current_user, db,
         user_id=user_id, type_id=type_id, status=status, year=year,
+        as_substitute=as_substitute,
     )
     return AbsenceListResponse(total=total, items=[AbsenceOut.model_validate(a) for a in absences])
 
@@ -325,6 +330,44 @@ async def cancel_absence(
     return AbsenceOut.model_validate(absence)
 
 
+@router.post("/absences/{absence_id}/request-cancellation", response_model=AbsenceOut)
+async def request_cancellation(
+    absence_id: UUID,
+    data: CancellationRequest,
+    current_user: CurrentUser,
+    db: AsyncSession = Depends(get_db),
+):
+    """Mitarbeiter beantragt Stornierung eines genehmigten Antrags (Manager genehmigt/lehnt ab).
+
+    HR/Admin storniert weiterhin direkt über DELETE."""
+    absence = await absence_service.request_cancellation(absence_id, data.reason, current_user, db)
+    await db.commit()
+    return AbsenceOut.model_validate(absence)
+
+
+# ── Kommentare ────────────────────────────────────────────────────────────────
+
+@router.get("/absences/{absence_id}/comments", response_model=list[AbsenceCommentOut])
+async def list_absence_comments(
+    absence_id: UUID,
+    current_user: CurrentUser,
+    db: AsyncSession = Depends(get_db),
+):
+    return await absence_service.list_comments(absence_id, current_user, db)
+
+
+@router.post("/absences/{absence_id}/comments", response_model=AbsenceCommentOut, status_code=201)
+async def add_absence_comment(
+    absence_id: UUID,
+    data: AbsenceCommentCreate,
+    current_user: CurrentUser,
+    db: AsyncSession = Depends(get_db),
+):
+    comment = await absence_service.add_comment(absence_id, data.body, current_user, db)
+    await db.commit()
+    return comment
+
+
 class AbsenceApproveOut(AbsenceOut):
     warnings: list[str] = []