refactor: migrate from ingress-nginx to Traefik v3

Replace ingress-nginx 4.15.1 with Traefik v3 (Helm chart 39.0.7)
as the ingress controller for the local kind cluster.

- Replace k8s/nginx/ with k8s/traefik/ (Helm chart, values, namespace)
- Update setup script selectors and namespace references
- Convert nginx upstream-vhost annotations to Traefik Middleware CRDs
- Update ingressClassName from nginx to traefik

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

k8s/app/local-proxy.yaml

@@ -24,9 +24,9 @@ kind: Ingress
 metadata:
   name: frontend
   annotations:
-    nginx.ingress.kubernetes.io/upstream-vhost: "localhost:3300"
+    traefik.ingress.kubernetes.io/router.middlewares: default-frontend-host@kubernetescrd
 spec:
-  ingressClassName: nginx
+  ingressClassName: traefik
   tls:
     - hosts:
         - staging-shiny.unbound.se
@@ -59,9 +59,9 @@ kind: Ingress
 metadata:
   name: api
   annotations:
-    nginx.ingress.kubernetes.io/upstream-vhost: "localhost:4444"
+    traefik.ingress.kubernetes.io/router.middlewares: default-api-host@kubernetescrd
 spec:
-  ingressClassName: nginx
+  ingressClassName: traefik
   tls:
     - hosts:
         - staging-shiny-api.unbound.se
@@ -77,3 +77,21 @@ spec:
                 name: api-external
                 port:
                   number: 4444
+---
+apiVersion: traefik.io/v1alpha1
+kind: Middleware
+metadata:
+  name: frontend-host
+spec:
+  headers:
+    customRequestHeaders:
+      Host: "localhost:3300"
+---
+apiVersion: traefik.io/v1alpha1
+kind: Middleware
+metadata:
+  name: api-host
+spec:
+  headers:
+    customRequestHeaders:
+      Host: "localhost:4444"