trmnl-image-relay
POST an image (PNG/JPEG, raw body or multipart) to / with Authorization: Bearer <token>.
It is resized to 800x480, dithered to the E Ink Spectra 6 palette (full color),
and forwarded to the configured TRMNL Webhook Image URL.
Params: fit=letterbox|crop  bg=black|white|RRGGBB  dither=auto|fs|atkinson|bayer|none
        sat=1.25  levels=0  target=50  dryrun=1
        stamp=0 disables the "last refreshed" chip; stamp_pos=tl|tr|bl|br; stamp_scale=1-4
GET /health for liveness.