fix: Python 3.10 compat for datetime.UTC in api_endpoints

datetime.UTC was added in Python 3.11. Fall back to timezone.utc on
older interpreters, matching the existing pattern in mqtt_handler.py.

Co-Authored-By: Zindello <josh@zindello.com.au>

repeater/web/api_endpoints.py

@@ -2,7 +2,13 @@ import json
 import logging
 import os
 import time
-from datetime import UTC, datetime
+from datetime import datetime
+
+try:
+    from datetime import UTC
+except ImportError:
+    from datetime import timezone
+    UTC = timezone.utc
 from pathlib import Path
 from typing import Callable, Optional