auto_backup_before_deploy silently did nothing — MaybeBackupBeforeDeploy's
only caller was the legacy executeDeploy pipeline, removed in the
workload-first cutover. Reconnect it as maybeBackupBeforeDeploy(), invoked
from DispatchPlugin after the source resolves and before it runs, so the
setting fires for every source kind. Fail-open: a nil backuper, a
settings-load error, or a backup failure skips the snapshot without
blocking the deploy. Adds predeploy_backup_test.go asserting the wiring.
