Improve kci-summary API performance

Discussed approaches without migrating the database:
- [ ] Limit the query by checkout numbers instead of days?
- [ ] Get history and summary and execute everything in python

On hold until we check for performance improvements with the database replica

The issue names the kci-summary API but no files or tests; first check performance improvements with the database replica, then evaluate the proposed checkout-number query and Python history-and-summary approaches without migrating the database. Done means selecting and implementing an approach with measurable performance improvement, though no target is specified.

Written by the indexing model from the issue text.