feat: deploy to Frostmoln production (#3100)

Part of the dancefinder.unbound.se cutover to Frostmoln Production (infra: unboundsoftware/infra#1896, already applied).

- Replaces the AWS `deploy-prod` job (failing since CI left AWS) with `deploy-prod-frostmoln` (`deploy prod-frostmoln -n dancefinder`), as schemas did.
- Restricted securityContext (runAsNonRoot, drop ALL, seccomp RuntimeDefault) and `imagePullSecrets: oci-registry` (Platform registry).
- Switches to `nginxinc/nginx-unprivileged` (same pin as schemas-app), listening on 8080 as uid 101; the Service keeps port 80. Traefik Ingress for `/`, PDB, HPA moved to `-prod-frostmoln`; ALB Ingress dropped; Service ClusterIP.

Merging deploys to Frostmoln only; DNS still points at AWS until the cutover.

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

https://claude.ai/code/session_01H6eVuR4Yt3m7TbDTDwyZJi
Reviewed-on: https://gitea.unbound.se/dancefinder/dancefinder-app/pulls/3100

nginx-conf/default.conf

@@ -1,5 +1,5 @@
 server {
-    listen       80;
+    listen       8080;
     server_name  localhost;
     sendfile     off;
     charset      utf-8;