Replace deprecated datetime.utcnow() with datetime.now(timezone.utc)

datetime.utcnow() is deprecated in Python 3.12 and removed in 3.13.
Replace all four call sites with timezone-aware equivalents so the
codebase is ready for Python 3.12+.

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

monitor.py

@@ -12,7 +12,7 @@ import logging
 import re
 import shlex
 import time
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Dict, List, Optional
 
 import requests
@@ -618,7 +618,7 @@ class LinkStatsCollector:
         return {
             'hosts':          result_hosts,
             'unifi_switches': unifi_switches,
-            'updated':        datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S UTC'),
+            'updated':        datetime.now(timezone.utc).strftime('%Y-%m-%d %H:%M:%S UTC'),
         }
 
     def _compute_unifi_rates(self, raw: Dict[str, dict], now: float) -> Dict[str, dict]:
@@ -653,7 +653,7 @@ class LinkStatsCollector:
 # Helpers
 # --------------------------------------------------------------------------
 def _now_utc() -> str:
-    return datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S UTC')
+    return datetime.now(timezone.utc).strftime('%Y-%m-%d %H:%M:%S UTC')
 
 
 # --------------------------------------------------------------------------
@@ -921,7 +921,7 @@ class NetworkMonitor:
         return {
             'hosts': hosts,
             'unifi': display_unifi,
-            'updated': datetime.utcnow().isoformat() + 'Z',
+            'updated': datetime.now(timezone.utc).isoformat().replace('+00:00', 'Z'),
         }
 
     # ------------------------------------------------------------------