Hide the old pobsync_home field from the Django admin and replace legacy
operator-facing labels with runtime state root and backup root terminology.
Rename admin compatibility fieldsets, update self-check/config-check text,
and refresh management command help so Django/systemd stays the primary
mental model.
Add linked admin summaries for hosts, snapshots, and backup runs so the SQL-first
backup state is easier to inspect from the Django admin. Hosts now link to their
filtered snapshot and run lists, backup runs link back to their snapshot, and
snapshots show base/run relationships without requiring filesystem inspection.
Cover the new admin display helpers with focused tests.
