List Cities

Cities nf has written ledgers for, with which stages each one holds.

A city with only old per-run objects and no canonical/ prefix is not listed: it has no nf ledgers, and showing it with every stage missing would read as a failed run rather than as a city the new pipeline has not touched.

ONE list of the whole root, not one per city. Listing each city's prefix separately measured at 0.25 s × 155 cities = 39 s for this endpoint; the root holds 2,518 objects and comes back in a second, so the filtering is cheaper done here than in S3.