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>

db.py

@@ -3,7 +3,7 @@ import json
 import logging
 import threading
 from contextlib import contextmanager
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Optional
 
 import pymysql
@@ -281,7 +281,7 @@ def create_suppression(
 ) -> int:
     expires_at = None
     if expires_minutes:
-        expires_at = datetime.utcnow() + timedelta(minutes=int(expires_minutes))
+        expires_at = datetime.now(timezone.utc) + timedelta(minutes=int(expires_minutes))
     with get_conn() as conn:
         with conn.cursor() as cur:
             cur.execute(