fix: normalize return risk timestamps

backend/app/api/routers/bookings.py

@@ -14,6 +14,7 @@ from app.schemas import (
     CurrentUser,
     NextBookingRisk,
     RegisterReturnRequest,
+    RegisterReturnResult,
     ReturnPreviewResult,
 )
 from app.services.returns import preview_vehicle_return, register_vehicle_return
@@ -133,7 +134,7 @@ def preview_return(
     )
 
 
-@router.post("/{public_ref}/return")
+@router.post("/{public_ref}/return", response_model=RegisterReturnResult)
 def register_return(
     public_ref: str,
     body: RegisterReturnRequest,
@@ -141,7 +142,7 @@ def register_return(
     idempotency_key: str = Header(..., alias="Idempotency-Key", min_length=8, max_length=128),
     db: Session = Depends(get_db),
     user: CurrentUser = Depends(get_current_user),
-) -> dict:
+) -> RegisterReturnResult:
     status_code, result = register_vehicle_return(db, public_ref, body, idempotency_key, user)
     response.status_code = status_code
-    return result
+    return RegisterReturnResult(**result)