style(live-state): drop unused imports staged for later tasks

threading, time, Callable were imported in anticipation of Task 9's
daemon-thread startup. Task 9 will re-add them when actually needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

l4d2web/services/live_state_poller.py

@@ -12,10 +12,7 @@ This file is built up across Tasks 6-10.
 from __future__ import annotations
 
 import logging
-import threading
-import time
 from datetime import datetime, UTC
-from typing import Callable
 
 from sqlalchemy import select