Timezone handling for DATETIME annotations is broken

HTML field produces "2026-03-23T23:48" without seconds or time zone.

`paikkalize_schedule_item` uses `parse_datetime`, which assumes UTC if time zone is not provided. I'm pretty sure that's not what the user would expect. Should use event timezone instead.

The annotation should always be in full ISO format with time zone. Can be stored in UTC as long as it's shown to user always in event timezone.

Coming back from the annotation to the HTML field, it should again be cnoverted to event timezone before being shown to user if stored in anything else.