Refactor response construction in sample endpoint.

Replaced direct return of `last_event` with an explicit construction of `SampleEventResponse`. This ensures clarity and better adherence to the response schema.

backend/app/routers/sample.py

@@ -99,7 +99,12 @@ async def get_last_sample_event(sample_id: int, db: Session = Depends(get_db)):
     if not last_event:
         raise HTTPException(status_code=404, detail="No events found for the sample")
 
-    return last_event  # Response will automatically use the SampleEventResponse schema
+    return SampleEventResponse(
+        id=last_event.id,
+        sample_id=last_event.sample_id,
+        event_type=last_event.event_type,
+        timestamp=last_event.timestamp,
+    )  # Response will automatically use the SampleEventResponse schema
 
 
 @router.post("/samples/{sample_id}/upload-images")