The two `x.isoformat() if x else None` copies in auth's device serializer now use the shared iso() helper — completes the isoformat-idiom sweep outside notes.py (auth + sync done; notes.py's remain, tied to its split). _serialize_user is unchanged (no datetime, no cross-module duplicate) and stays in auth.py rather than relocating for no DRY gain. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm