Enh: option to limit stats.lua output by message count

The stats.lua API is not really suited to mobile devices which may have limited data allowances (or limited memory).
A mobile user is not likely to want to scroll through a month's worth of data.
It's more likely that they will want to review the most recent messages in a few mailing lists.

Whilst the results can be limited by start and end date, even a single day can hold too many messages.

It would be useful to be able to limit the response by size, so the user could request the first N messages going backards from a specific start time (default now ).

Start by reading stats.lua to understand its existing start and end date handling and how a response is assembled. Trace the API entry point and relevant tests, then define the count and default-start-time behavior; done means callers can request the most recent N messages going backward from a specified start time without changing existing date-limited behavior.

Written by the indexing model from the issue text.