PaperColor dashboard worker

The M5Stack PaperColor firmware GETs /render.json on each wake,
interprets the Op[] with M5GFX, draws to the Spectra 6 panel, then
deep-sleeps for nextWakeSec. The server owns the cadence.

Read endpoints (no auth):
  GET  /render.json   -> { version, w, h, palette, nextWakeSec, ops }
  GET  /preview       -> HTML/SVG mockup at 400x600 (browser iteration)
  GET  /manage        -> HTML/JS UI for managing todos (asks for token)
  GET  /todos         -> list current todos as JSON
  GET  /healthz       -> ok

Write endpoints (require Authorization: Bearer <TODO_WRITE_TOKEN>):
  POST   /todo        -> body {"text":"...","id":"optional-stable-id"}
  DELETE /todo/:id    -> remove one
  DELETE /todos       -> clear all