perf: optimize system concurrency, I/O stability and fix WebSocket disconnects

backend/api/guard.py

@@ -7,7 +7,7 @@ Provides REST API endpoints for tool guard operations.
 from __future__ import annotations
 
 from typing import Any, Dict, List, Optional
-from datetime import UTC, datetime
+from datetime import datetime, timezone
 
 from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel, Field
@@ -146,7 +146,7 @@ async def check_tool_call(
 
     if request.tool_name in SAFE_TOOLS:
         record.status = ApprovalStatus.APPROVED
-        record.resolved_at = datetime.now(UTC)
+        record.resolved_at = datetime.now(timezone.utc)
         record.resolved_by = "system"
         STORE.set_status(
             record.approval_id,