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

backend/agents/base/tool_guard.py

@@ -12,7 +12,7 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
-from datetime import UTC, datetime
+from datetime import datetime, timezone
 from enum import Enum
 
 from typing import Any, Callable, Dict, Iterable, List, Optional, Set
@@ -78,7 +78,7 @@ class ApprovalRecord:
         self.session_id = session_id
         self.status = ApprovalStatus.PENDING
         self.findings = findings or []
-        self.created_at = datetime.now(UTC)
+        self.created_at = datetime.now(timezone.utc)
         self.resolved_at: Optional[datetime] = None
         self.resolved_by: Optional[str] = None
         self.metadata: Dict[str, Any] = {}
@@ -163,7 +163,7 @@ class ToolGuardStore:
             return record
 
         record.status = status
-        record.resolved_at = datetime.now(UTC)
+        record.resolved_at = datetime.now(timezone.utc)
         record.resolved_by = resolved_by
         if notify_request and record.pending_request:
             if status == ApprovalStatus.APPROVED: