Negative credits in usage list view

[jldec]

Something is wrong with the minutes -> credits computation.

[laushinka]

I investigated a particular workspaceId which was showing -0.4. The frontend received the negative amount from the server, while the credits entry persisted in d_b_workspace_instance_usage was not negative.
After a few minutes of further investigation, I returned to the usage list, which now shows the correct number! The payload received by the frontend is also correct.
Apparently these negative values get corrected eventually, but how they came to be negative values in the first place still requires further investigation in the API, i.e. where we compute the credit cost.

I will unassign myself for now to allow others to pick it up, since I'll be focusing on other tickets.

[geropl]

@laushinka and I synced on this one:

• we figure out that the DB stores negative values indeed, so we assume it's not the retrieval path that introduces these values
• instead, we assume that GenerateUsageReport is wrong due false assumptions about timezones of it's parameters

I will try to produce a test and fix it.