pxe: reload dnsmasq on host create/delete

pxe.Supervisor.Reload() was defined but never wired up. After a host
was registered in the UI or via the quick-register JSON endpoint, the
dnsmasq conf still held only the hosts that existed at orchestrator
startup. The new MAC wasn't tagged `known`, so when the host PXE'd,
dnsmasq logged "PXE(eth0) <mac> proxy-ignored" and the boot timed out
back to the BIOS.

Add an optional PXEReloader interface to api.UI, wire it from main
when pxe is enabled, and call u.reloadPXE() after successful Create
and Delete. Logs-and-continues on failure — host registration itself
has already committed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/vetting/main.go

@@ -157,6 +157,7 @@ func main() {
 			TFTPRoot:        tftpRoot,
 			LiveDir:         cfg.PXE.LiveDir,
 		})
+		ui.PXE = supervisor
 	}
 
 	router := httpserver.NewRouter(httpserver.Deps{