forgepulse: the 1d/7d/30d windows now count back from the newest day GitHub reported

The window columns in forgepulse were mostly empty at their recent end: GitHub’s traffic API lags a day or more, so counting back from the wall clock left the newest days without rows and the 1d column read zero for nearly every repository (7f5acb4). The windows now count back from the newest day that at least half of all repositories have a row for, and each window is capped at that day, so a stray early same-day row cannot leak in as a partial day or move the anchor — a plain MAX(day) would let one early reporter blank out everyone else. It also makes 1d genuinely one day: the old >= now − 1 day spanned up to two. It is the same lag the per-repository clone baselines were added for, now fixed at the window itself rather than at the comparison.