Fix serialization of datetimes to include Z suffix

[tw4l]

trafficstars

We store datetimes in UTC but they are not returned with a Z suffix from the API, which requires us to manually add it in the frontend.

We should modify serialization to return timezone from the API. See: https://stackoverflow.com/questions/66548586/how-to-change-date-format-in-pydantic

Ideally we could do this globally so we don't need to modify every class with a datetime.

The PR should also update the frontend where a Z is being added manually.