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

backend/runtime/manager.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 import asyncio
 import json
-from datetime import datetime, UTC
+from datetime import datetime, timezone
 from pathlib import Path
 from typing import Any, Dict, List, Optional
 
@@ -93,7 +93,7 @@ class TradingRuntimeManager:
 
     def log_event(self, event: str, details: Optional[Dict[str, Any]] = None) -> Dict[str, Any]:
         entry = {
-            "timestamp": datetime.now(UTC).isoformat(),
+            "timestamp": datetime.now(timezone.utc).isoformat(),
             "event": event,
             "details": details or {},
             "session": self.current_session_key,
@@ -120,7 +120,7 @@ class TradingRuntimeManager:
 
     def register_pending_approval(self, approval_id: str, payload: Dict[str, Any]) -> None:
         payload.setdefault("status", "pending")
-        payload.setdefault("created_at", datetime.now(UTC).isoformat())
+        payload.setdefault("created_at", datetime.now(timezone.utc).isoformat())
         self.pending_approvals[approval_id] = payload
         self._persist_snapshot()
 
@@ -149,7 +149,7 @@ class TradingRuntimeManager:
         if not entry:
             return
         entry["status"] = status
-        entry["resolved_at"] = datetime.now(UTC).isoformat()
+        entry["resolved_at"] = datetime.now(timezone.utc).isoformat()
         entry["resolved_by"] = resolved_by
         self._persist_snapshot()