docs: complete optional status app setup
The optional status-app instructions now include the build and start commands, required tools, and port 3002. This completes #808’s self-host URL guidance and corrects the claim that Bun and Node are only needed for local development.
Validation: repository lint, all 33 typecheck tasks, and all 29 test tasks pass. The documented production build and start were exercised with synthetic local URLs: /health returned 200 and robots.txt used the configured status URL. All 14 local links/anchors and code fences pass. README only; no runtime changes.
Summary by cubic
Completes the optional status app setup guidance from #808. The README now shows the required Node, Bun, and dependencies, the production build and start commands, and port 3002, correcting the earlier claim that Bun and Node were only needed for local development; docs-only change, so no runtime impact.