Billing — multi-yaml/flows/billing

nodes:
  - {id: in, type: http_trigger, path: /billing, methods: [POST]}
  - {id: rate, type: rate_limit, store: local, rpm: 1200}
  - {id: body, type: read_body, max_bytes: 1048576}
  - {id: auth, type: auth, scheme: exact, secret: "change-me-demo-token", signature_header: X-Auth}
  - {id: env, type: envelope, body_format: json}
  - {id: reply, type: respond, status: 202, body: "accepted billing"}
edges: [in>rate, rate>body, body>auth, auth>env, env>reply]