fix: resolve 57 ruff lint errors (unused imports, unused variables, import placement)

tests/test_auth_api.py

@@ -9,7 +9,8 @@ import pytest
 @pytest.fixture
 def auth_client():
     """Create a TestClient with auth enabled, isolated temp data."""
-    import tempfile, shutil
+    import tempfile
+    import shutil
     from pathlib import Path
 
     tmp = Path(tempfile.mkdtemp())
@@ -58,7 +59,7 @@ def auth_client():
     backend.main._load_config = lambda: {"watcher_enabled": False}
 
     from backend.main import app
-    from backend.indexer import build_index, vault_config, index
+    from backend.indexer import build_index, index
     import asyncio
 
     for key in list(index.keys()):