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 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

@@ -1,4 +1,5 @@
-FROM amd64/nginx:1.31.6@sha256:9a54e7e76ae7fb26e733c7d43ca1c8586f1894b876ec8b8ca0124daae38d509b
+# Unprivileged (uid 101, port 8080): Frostmoln enforces restricted Pod Security.
+FROM nginxinc/nginx-unprivileged:1.31.6@sha256:1d26a516fb93b18f9c0214dc9c645054486e516f13ff09ef63d5bd5c622278a2
 MAINTAINER Joakim Olsson <joakim@unbound.se>
 
 ADD files/*.html files/*.jpg files/*.mp3 /usr/share/nginx/html/