feat: deploy to Frostmoln production (#243)

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: #243

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