Kindle dashboard worker

Read endpoints (no auth):
  GET  /render.sh     -> shell script of fbink commands (curl | sh from Kindle)
  GET  /render.json   -> same composition as JSON ops
  GET  /preview       -> HTML/SVG mockup at 758x1024 (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

Note: every /render.sh now does a full e-ink flash refresh by default,
since cron runs once a day and ghosting accumulation isn't an issue.