fix(absences): POST /absences/ reicht warnings jetzt an den Client durch

create_absence() gibt (absence, warnings) zurück, der Router verwarf die
Warnungen bisher (im Unterschied zu approve_absence, das sie via
AbsenceApproveOut.warnings durchreicht). Neues AbsenceCreateOut-Schema
analog dazu. Regressionstest über Überschneidungs-Warnung.

Gefunden während Coverage-Arbeit an absence_service.py.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Ahyx6D3r7G1EuAc42nezn

backend/app/routers/absences.py

@@ -262,7 +262,11 @@ async def list_absences(
     return AbsenceListResponse(total=total, items=[AbsenceOut.model_validate(a) for a in absences])
 
 
-@router.post("/absences/", response_model=AbsenceOut, status_code=201)
+class AbsenceCreateOut(AbsenceOut):
+    warnings: list[str] = []
+
+
+@router.post("/absences/", response_model=AbsenceCreateOut, status_code=201)
 async def create_absence(
     data: AbsenceCreate,
     current_user: CurrentUser,
@@ -281,7 +285,9 @@ async def create_absence(
         acting_user = target
     absence, warnings = await absence_service.create_absence(data, acting_user, db)
     await db.commit()
-    return AbsenceOut.model_validate(absence)
+    out = AbsenceCreateOut.model_validate(absence)
+    out.warnings = warnings
+    return out
 
 
 @router.patch("/absences/{absence_id}", response_model=AbsenceOut)