Add a stats summary layer that aggregates recent successful real backup runs
into dashboard and host-level trend metrics.
Show backup-root usage, available space, average new data, average duration,
estimated runs until full, and link-dest savings on the dashboard. Add a host
trend table with recent run duration, file count, new data, matched data, and
snapshot links.
Keep the implementation based on existing run and snapshot stats JSON so the
UI gains useful trend visibility without introducing a schema migration yet.
