fix(upload): missing asyncio import — module-level Lock() fired NameError on import

backend/uploaded_docs.py used asyncio.Lock() + asyncio.wait_for but
never imported asyncio. The module-level _UPLOAD_EXTRACTION_LOCK =
asyncio.Lock() raised NameError at import time, which surfaced as
"Indexing failed: NameError: name 'asyncio' is not defined" on every
POST /api/upload-policy after dfaa4d6.

Added `import asyncio` at the top of backend/uploaded_docs.py.

Verified live by re-importing the module under the project venv.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/uploaded_docs.py

@@ -56,6 +56,7 @@ ONE doc must not abort boot, but the failure is logged loudly).
 
 from __future__ import annotations
 
+import asyncio
 import hashlib
 import json
 import logging