Add a "new regressions" column to the Trees listing page

Part of #1972.

## Summary

Add a "New regressions" column to the Trees listing page, showing the count of new regressions per row.

## Acceptance criteria

- [ ] The Trees listing shows a new-regressions column with an accurate count.
- [ ] No noticeable increase in listing load time from the added data.

Start at the Trees listing page and trace how each row's data is loaded, then identify the existing regression-count data path. Add the New regressions column and verify that each row shows an accurate count without a noticeable increase in listing load time.

Written by the indexing model from the issue text.