Migrate to modern datetime library API

Signed-off-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>

backend/utils/logger.py

@@ -12,7 +12,7 @@ import logging
 import json
 import sys
 import os
-from datetime import datetime
+from datetime import datetime, timezone
 from contextvars import ContextVar
 from functools import wraps
 import traceback
@@ -29,7 +29,7 @@ class JSONFormatter(logging.Formatter):
     def format(self, record: logging.LogRecord) -> str:
         """Format log record as JSON with contextual information."""
         log_data = {
-            'timestamp': datetime.utcnow().isoformat(),
+            'timestamp': datetime.now(timezone.utc).replace(tzinfo=None).isoformat(),
             'level': record.levelname,
             'message': record.getMessage(),
             'module': record.module,