Get project factory velocity

Counts this project's factory PRs that merged, by merge time, 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, and the tickets they finished. A factory PR is one the project’s runs produced, eval PRs left out. A PR’s tickets are the GitHub and Linear issues it was made for. A ticket is finished when its first merged factory PR merges, and counts once, in the window of that merge. trackedSince is when the repository’s record of PRs starts, null before its first. series has one point per UTC day that the current window touches, zeros included, and its points add up to current.