from datetime import datetime, timezone


def isoformat_utc(dt: datetime | None) -> str | None:
    """Serialize a datetime for API responses, stamping it UTC first.

    Every DateTime column is written via app.db.models.utcnow() but SQLite/aiosqlite
    round-trips it as a naive datetime, so a bare .isoformat() drops the "Z"/offset
    and JavaScript's `new Date()` on the frontend parses the result as local time
    instead of UTC (B-35). All stored values are UTC in practice, so a naive value
    can be safely stamped rather than converted.
    """
    if dt is None:
        return None
    if dt.tzinfo is None:
        dt = dt.replace(tzinfo=timezone.utc)
    return dt.isoformat()