Fix Mongo default database fallback

app/db.py

@@ -54,7 +54,8 @@ def fetch_recent_transactions(user_id: str, months: int = 3) -> List[Transaction
         return []
 
     client = get_mongo_client()
-    db = client.get_default_database() or client["expense"]
+    default_db = client.get_default_database()
+    db = default_db if default_db is not None else client["expense"]
     collection = db["transactions"]
 
     user_obj_id = _safe_object_id(user_id)