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

k8s/ingress-prod.yaml

@@ -1,23 +0,0 @@
-apiVersion: networking.k8s.io/v1
-kind: Ingress
-metadata:
-  name: lingonpaj-site-ingress
-  annotations:
-    kubernetes.io/ingress.class: "alb"
-    alb.ingress.kubernetes.io/group.name: default
-    alb.ingress.kubernetes.io/scheme: internet-facing
-    alb.ingress.kubernetes.io/target-type: instance
-    alb.ingress.kubernetes.io/listen-ports: '[{"HTTP": 80},{"HTTPS": 443}]'
-    alb.ingress.kubernetes.io/ssl-redirect: "443"
-spec:
-  rules:
-    - host: "lingonpaj.com"
-      http:
-        paths:
-          - path: /
-            pathType: Prefix
-            backend:
-              service:
-                name: lingonpaj-site
-                port:
-                  number: 80