Datetime format is inconsistent

[frankie567]

The datetime values we generate ourselves include the fractional seconds. The ones we get directly from Stripe don't.

Technically, this is not a problem for PostgreSQL or Pydantic. However, in API output, we might return datetimes in either formats, which might add difficulties for parsing in some languages.

There is apparently no obvious way to globally configure Pydantic datetime serialization format, so I'm not sure yet how to tackle this.