Enh: provide caching info for results that may be cached

It might be useful to add some caching details to the responses that are cached.

For example:
cached: true/false
cached_at: timestamp (not nowish)
granularity: seconds between cache stores

This can help in testing.
I was sometimes getting a successful response from the server even after I closed the index.
Turns out it was that some sessions had cached the results; new ones had not.

Start by tracing where cached results are assembled and returned in the server response path. Check how cache timestamps and storage intervals are represented, then verify that responses expose whether results were cached, when they were cached, and the cache-store granularity so cached and fresh sessions can be distinguished.

Written by the indexing model from the issue text.