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[?screen=N] -> contract { ..., ops, screenIndex, screenCount }
  GET  /preview[?screen=N]     -> HTML/SVG mockup at 400x600
  GET  /manage        -> HTML/JS UI for managing todos (asks for token)
  GET  /photos        -> HTML/JS UI for the photo frame (asks for token)
  GET  /screens       -> HTML/JS UI for the screen playlist (asks for token)
  GET  /data          -> HTML/JS UI to edit/review data-screen cards (asks for token)
  GET  /todos         -> list current todos as JSON
  GET  /screens.json  -> the screen playlist as JSON
  GET  /images        -> list photo ids as JSON
  GET  /image/:id     -> processed JPEG (400×600, from R2)
  GET  /data/:slug    -> a pushed card payload 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
  PUT    /screens     -> body {"screens":[...]} (replace the playlist)
  DELETE /screens     -> reset playlist to default
  POST   /image       -> raw image body or multipart; processed -> R2
  DELETE /image/:id   -> remove one photo
  DELETE /images      -> clear all photos
  PUT    /data/:slug  -> body {"title?","sections":[{"heading?","items":[...]}]}
  DELETE /data/:slug  -> clear that card