Keeps this otherwise-empty directory tracked in git.

Each business gets its own <slug>.caddy file here, e.g.:

  demo-business.example.com {
  	reverse_proxy next-chair-demo-business:3000
  }

Generated by the management UI (manager/) when a business is added, or
written by hand for the by-hand setup steps in stacks/infra/compose.yaml's
header comment. Real *.caddy files are gitignored (deployment-specific,
not something to hand-author into the repo) — only this placeholder is
tracked.