Add missing imports and clear backlink index

Resolve build index regression causing stale backlink data on reindex.

backend/auth/jwt_handler.py

@@ -4,6 +4,7 @@
 # Revoked token JTIs persisted to data/revoked_tokens.json.
 
 import json
+import os
 import secrets
 import uuid
 import time

backend/indexer.py

@@ -332,6 +332,7 @@ async def build_index(progress_callback=None) -> None:
         index.clear()
         _file_lookup.clear()
         path_index.clear()
+        _backlink_index.clear()
         _index_generation += 1
 
     if not vault_config: