docker: an update clears "update available" without a page reload

The registry's answer was cached and served as-is, so the stack view and the
machine-wide list kept calling an image behind after an update had pulled it.
The cache now keeps the digest the registry named, and an image marked behind
is re-checked locally on every read: once this machine holds that digest it
is current, at no registry cost. The page re-reads the machine-wide answer
after a stack task, a queued update, or a schedule run.

Also: stack keys can no longer collide with the machine key, and the dead
"/"+stack forget in the schedule runner is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/dockersched.go

@@ -340,7 +340,6 @@ func (s *Server) dsRun(w http.ResponseWriter, r *http.Request) {
 		rec.Error = err.Error()
 	}
 	_ = storage.WriteRun(s.cfg.DataDir(), "docker-"+id, rec)
-	stackUpdatesForget("/" + job.Stack)
 	if err != nil {
 		jsonErr(w, 500, err.Error()+"\n"+tail(out, 2000))
 		return