Fix getRecentTraces sorting/limiting logic

Collect all summaries before sorting and limiting to ensure we return
the actual most recent traces, not just the first N traces returned
by KvStore.list().

Also removed unnecessary type annotation on summaryPrefix.

https://github.com/fedify-dev/fedify/pull/502#discussion_r2642993269
https://github.com/fedify-dev/fedify/pull/502#discussion_r2642989826