Get project factory utilization

Counts the PRs opened in the project's repository, from GitHub webhooks, in the requested window (filter[timestamp][gte] to the exclusive filter[timestamp][lt], at most 90 days, the last 7 days when neither is given) and in the window of the same length just before it, by open time. factoryPrs counts those a launch run produced. Counts cover the whole repository, whatever the project’s subpath. trackedSince is when the repository’s record starts, null before its first PR; earlier PRs are not counted. series has one point per UTC day that the current window touches, zeros included, and its points add up to current.