[REFACTOR]: improve date handling of activity api

[jon4hz]

What to refactor

At the moment, the timed API for starting and stopping activities takes a date, from-time and to-time as input. The date contains only a day like 2025-06-06 and the from-time and to-time contain a timestamp without date.

Why refactor it

The activity API should be able to handle tasks that end on a different day than they were started.
Currently, the frontend somehow handles this gracefully. Without going through the code, I'd assume it stops the activity at 23:59:59 and starts a new activity for the next day.

However, that seems a bit like a workaround. And every third party client that interacts with the activity API has to reimplement that logic.

How to refactor it

I think it would make sense to refactor this so from-time and to-time are both ISO 8601 timestamps, containing a date and time. That makes it easier to handle activities that are stopped on the next day.

Required

• I have checked past issues to ensure that this isn't a duplicate.

[jon4hz]

I just ran into an issue with our metabase instance that might be related to this.

According to this issue metabase doesn't handle timestamps without timezone very well.

If you are every going to pick up this issue, it would be nice if you could also make sure, that all timestamps have a timezone.