Billing hourly usage

Currently the only way to view usage is by day, in some period.

We want to be able to see usage in more detail, on an hourly basis, or even more granular.

API responses return usage by day only, this needs to add it to the frontend and backend

Start by tracing the existing day-based usage API and the frontend view that consumes it; the payload does not name specific files or tests. Done means usage can be returned and displayed at hourly or finer granularity across both the backend and frontend.

Written by the indexing model from the issue text.