Artifact sweep keeps digests named in node settings

A Service's weights or a wren node's model held only in a setting were
swept once the run that produced them was deleted.

docs/code/api.md

@@ -180,9 +180,10 @@ superusers may see answers 403 to anyone else for the frames its messages
 hold, the last 8 included (see [Who may watch a camera](../interface/dashboards.md#who-may-watch-a-camera)).
 
 A superuser can remove stored bytes with `DELETE /artifacts/<digest>`. While
-a run, a current message or a flow input's initial value still references the
-digest, the call returns `409`. The hourly sweep removes unreferenced bytes
-on its own, so this is for reclaiming space sooner.
+a run, a current message, a flow input's initial value or a node setting in a
+published or draft flow still references the digest, the call returns `409`.
+The hourly sweep removes unreferenced bytes on its own, so this is for
+reclaiming space sooner.
 
 ## Dashboards and devices
 

docs/code/workers.md

@@ -118,6 +118,8 @@ Set `--service-host` on the worker where that address is wrong.
 
 A service's `weights` setting holds an artifact digest. The worker fetches it
 as node code fetches an artifact and passes the local path as `{weights}`.
+The hourly artifact sweep keeps any digest a node setting names in a published
+or draft flow, so the weights stay after the run that produced them is deleted.
 
 ## Resources and artifacts