Show keep/delete reasons in the retention plan, surface scheduled prune
limit warnings, and explain base snapshot protection before retention is
applied.
Also surface incomplete snapshots from the retention views without deleting
them automatically, so interrupted backups are visible on the dashboard,
host detail, and retention plan.
Make SQL retention delete the snapshot root when records point at the
snapshot data directory, matching how backup metadata is stored on disk.
Before removing a snapshot tree, temporarily add user write permission to
directories inside that snapshot so rsync-preserved source permissions do
not block cleanup.
Add a regression test for pruning snapshots whose data directory mirrors a
read-only remote root.
