The controller writes the service's return value into the response, but
createMonitor was typed Promise<void> and returned nothing, so every
successful POST /monitors answered data: null and callers had no way to
read the new monitor's id.
Fixes#3917
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D7eViZ2Np5ZpWUYNoRhmAt