Fix missing Optional import in compiler.py

backend/compiler.py

@@ -1,6 +1,6 @@
 # Article compilation module - Groups and merges related articles
 
-from typing import List, Dict, Tuple
+from typing import List, Dict, Tuple, Optional
 from sklearn.feature_extraction.text import TfidfVectorizer
 from sklearn.metrics.pairwise import cosine_similarity
 from loguru import logger