ledboard 0.1.0
POST /text  with a plain-text body or {"text": "...", "color": "#hex"}
            "duration_s": seconds to show it, max 60
            Authorization: Bearer <clerk jwt> when LEDBOARD_AUTH_ISSUER is set
GET  /etch  etch-a-sketch state (background layer, bus/text overwrite it)
POST /etch/move {"dx": 1, "dy": 0}  turn the knobs, draws live
POST /etch/clear  shake to wipe
GET  /sim   to watch the board in a browser
GET  /healthz