Cloud routes: use timezone.utc, not datetime.UTC

The app supports Python 3.10 (requires-python, and the base image is
python:3.10); datetime.UTC only exists from 3.11. Every other module already
uses timezone.utc — this was the one file out of step, and it only passed
locally because the dev venv is 3.13. The container would not import at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XtBzdDyLsmDaF1W7DLYtYM

backend/app/api/routes/cloud.py

@@ -9,7 +9,7 @@ from __future__ import annotations
 
 import asyncio
 import logging
-from datetime import UTC, datetime
+from datetime import datetime, timezone
 from typing import Any
 
 import httpx
@@ -105,7 +105,7 @@ async def enroll(
         # talking to. Nothing refreshes this.
         jwks=data["jwks"],
         local_user_id=str(current_user.id),
-        enrolled_at=datetime.now(UTC).isoformat(),
+        enrolled_at=datetime.now(timezone.utc).isoformat(),
         portal_account=current_user.email,
     )
     cloud_config.save(config)