ci: publish multi-arch image to GHCR; move status port to 4733

GitHub Actions builds linux/amd64 and linux/arm64 on every push to main and
pushes a single :latest tag to ghcr.io, so the container can be run without
cloning or building. Tests gate the publish step.

Dockerfile

@@ -8,7 +8,7 @@ COPY src/ src/
 
 ENV STATE_PATH=/data/state.json
 VOLUME /data
-EXPOSE 8080
+EXPOSE 4733
 
 # --env-file-if-exists so the container works with plain env vars when no .env is mounted.
 CMD ["node", "--env-file-if-exists=.env", "src/index.js", "--loop"]