feat: deploy to Frostmoln production

The AWS deploy-prod job has failed since CI left AWS (no k8s.unbound.se
context on the runners). Deploys now go to the Frostmoln production
cluster (namespace sites), the way Shiny deploys to Frostmoln staging.

Frostmoln enforces restricted Pod Security, so the served image is
nginx-unprivileged (uid 101, port 8080) with a matching securityContext.
The ALB ingress becomes a Traefik one with a Let's Encrypt certificate.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ma8eHhW59hLAJuLzBnLvUu

Dockerfile

@@ -13,7 +13,8 @@ COPY ./ /site
 WORKDIR /site
 RUN hugo
 
-FROM amd64/nginx:1.31.6-alpine@sha256:5f4b1cbc2fac6baad5fd73efa91aef5f40ee315cacc5af719402ae21bb573776
+# Unprivileged (uid 101, port 8080): Frostmoln enforces restricted Pod Security.
+FROM nginxinc/nginx-unprivileged:1.31.6-alpine@sha256:884e0b7f42f303641b0f4e78755aea647521dca0b89ca3be7992ad78b5c07434
 ADD default.conf /etc/nginx/conf.d/default.conf
 COPY --from=build /site/public /usr/share/nginx/html