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

Part of #1972.

## Summary

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

## Acceptance criteria

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

Start at the Labs listing page implementation and trace how each row's existing data is assembled, using issue #1972 for surrounding context. Add the new-regressions column with an accurate per-row count, then verify the listing remains responsive and meets both acceptance criteria.

Written by the indexing model from the issue text.