Replace deprecated datetime.utcnow() with timezone-aware UTC

Use datetime.now(timezone.utc) instead of deprecated utcnow().

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/email_monitor.py

@@ -2,7 +2,7 @@
 
 import asyncio
 import logging
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Optional
 
 from imapclient import IMAPClient
@@ -275,7 +275,7 @@ class EmailMonitor:
     ) -> tuple[bool, Optional[str]]:
         """Send webhook notification for a new email."""
         payload = EmailNotificationPayload(
-            timestamp=datetime.utcnow(),
+            timestamp=datetime.now(timezone.utc),
             email_id=email.id,
             mailbox=mailbox,
             from_email=email.from_address.email,
@@ -353,7 +353,7 @@ class EmailMonitor:
                 subject=email.subject,
                 email_date=email.date,
                 notification_sent=notification_sent,
-                notification_sent_at=datetime.utcnow() if notification_sent else None,
+                notification_sent_at=datetime.now(timezone.utc) if notification_sent else None,
             )
             session.add(seen)
             await session.commit()
@@ -375,7 +375,7 @@ class EmailMonitor:
             seen = result.scalars().first()
             if seen:
                 seen.notification_sent = success
-                seen.notification_sent_at = datetime.utcnow() if success else None
+                seen.notification_sent_at = datetime.now(timezone.utc) if success else None
                 seen.notification_error = error
                 session.add(seen)
                 await session.commit()