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.
Introduce reusable configuration checks for global settings and effective
host runtime configuration. The checks now surface risky backup settings
such as missing recursive rsync args, missing critical root excludes,
invalid SSH settings, missing credentials, and retention gaps.
Show these checks on the global config form, host edit form, and host
detail page so operators can validate the compounded host/global config
before starting real backup runs.
