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

backend/runtime/agent_runtime.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass, field
-from datetime import datetime, UTC
+from datetime import datetime, timezone
 from typing import Any, Dict
 
 
@@ -11,12 +11,12 @@ class AgentRuntimeState:
     display_name: str | None = None
     status: str = "idle"
     last_session: str | None = None
-    last_updated: datetime = field(default_factory=lambda: datetime.now(UTC))
+    last_updated: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
 
     def update(self, status: str, session_key: str | None = None) -> None:
         self.status = status
         self.last_session = session_key
-        self.last_updated = datetime.now(UTC)
+        self.last_updated = datetime.now(timezone.utc)
 
     def to_dict(self) -> Dict[str, Any]:
         return {