feat: deploy to Frostmoln production (#86)

Moves the production deploy from AWS to the Frostmoln production cluster (namespace `sites`).

- `deploy-prod` (AWS) has failed since CI left AWS: the runners have no `k8s.unbound.se` context, so the site on AWS still runs an older image. It's replaced by `deploy-prod-frostmoln`, the same pattern as Shiny's `deploy-frostmoln-staging`: `FROSTMOLN_PRODUCTION_DEPLOY_KEY` + `FROSTMOLN_PRODUCTION_CLUSTER_ID` (org), `BUILDTOOLS_FROSTMOLN_PRODUCTION` (org, target `prod-frostmoln`), `deploy prod-frostmoln -n sites`.
- Frostmoln enforces restricted Pod Security: the served image is `nginxinc/nginx-unprivileged` (uid 101, port 8080), with a matching securityContext; the Service is ClusterIP.
- `ingress-prod.yaml` (ALB) → `ingress-prod-frostmoln.yaml` (Traefik, Let's Encrypt).

Checked locally: the image serves `/health` and `/` (200) as uid 101 with all capabilities dropped.

**Merge after** infra's `feat/frostmoln-production-sites` is applied (namespace + org secret). The hostnames keep pointing at AWS until the Route 53 switch, a separate step; the certificate is issued after that.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01Ma8eHhW59hLAJuLzBnLvUu
Reviewed-on: #86

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/