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