Add a warning status for BackupRun records so successful snapshots are not
reported as failed when post-run SQL retention fails.
Keep the prune error in the run result, link the successful snapshot, and
let the management command complete with a warning instead of raising a
backup failure.
Include warning runs in backup trend summaries and add a regression test
for successful backups with failed retention cleanup.
Peter van Arkel
8bff241b12