fix: painfully slow statistics

[arcstur]

when the page was created it was rendering faster, now it takes so long!

maybe we can cache it every midnight? we don't have any jobs/crons available, so we would have to implement this

or increase the page cache from 10min to 2h, for example

[egonw]

I would also think the summaries needed for everything older than a week (or even day) should be a short list of counts, not? And if that is not enough, perhaps default to sums per week?