radar — post readings, get an instrument

  A server has no antenna and neither does an agent, so this half does not
  sense anything. POST signal readings and it draws them: concentric bands by
  strength, a meter and a trend per source.

    node tools/photos/radar-sample.ts --at https://aadhar.sh
    curl -X POST aadhar.sh/radar -d '{"samples":[{"name":"AP","rssi":-58}]}'

  ── the shape ───────────────────────────────────────────────────────────────
  name      whatever you want on the label
  rssi      dBm, negative; anything else is dropped rather than fatal
  kind      optional tag, e.g. wifi or ble
  history   optional trailing readings, for the trend sparkline

  ── what it will not pretend to know ────────────────────────────────────────
  RSSI is a scalar: it carries distance-ish information and no bearing. The
  rings are real; the angles are a hash of the name, stable so nothing jumps
  between frames, and meaningless. Bands are findphone's field calibration.

  Nothing is stored. Post the whole set each time — which also means device
  names travel in the request, so use --anonymize when pointing the sampler at
  a public host.

  POST readings  40 max sources