Allow BM25_INDEX_DIR override for quick testing

app.py

@@ -9,8 +9,8 @@ from core_logic import (
 )
 
 # Path to the index directory where the Hugging Face Storage Bucket is mounted.
-# Assuming the bucket is mounted at /data in the Space settings.
-INDEX_DIR = '/data/bm25_indexes'
+# Override with BM25_INDEX_DIR for local testing without code edits.
+INDEX_DIR = os.getenv('BM25_INDEX_DIR', '/data/bm25_indexes')
 
 def get_available_indices():
     if os.path.exists(INDEX_DIR):