thumbs: a GPU-access run already in flight refuses a second (409)

The sweep's battery hit the route three times; the second and third runs only collided
with the first's restart timer.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/api/thumbs.go

@@ -403,6 +403,10 @@ func (s *Server) thumbsGPUAccess(w http.ResponseWriter, r *http.Request) {
 		jsonErr(w, 400, "nothing to do: Ham Lab is root, has no root, or is not under systemd here")
 		return
 	}
+	if t, busy := s.tasks.RunningOf("thumbs:gpu"); busy { // a second run would only collide with the first's restart timer
+		jsonErr(w, 409, "already being done (task "+t.ID+")")
+		return
+	}
 	full := priv.Wrap("sh", "-c", gpuAccessScript)
 	t, err := s.tasks.Start("thumbs:gpu", "give Ham Lab access to the GPU", exec.Command(full[0], full[1:]...))
 	if err != nil {