Update app.py

app.py

@@ -527,7 +527,7 @@ async def get_processing_results(session_id: str):
             chunk["_id"] = str(chunk["_id"])
             
         # --- START FIX ---
-        # Convert potential datetime object to an ISO 8601 string
+        # Convert potential datetime object to an ISO 8601 string to make it JSON serializable
         processing_completed_at = session.get("processing_completed_at")
         if processing_completed_at:
             processing_completed_at = processing_completed_at.isoformat()