feat(k8s): Helm chart for the whole stack on a single-node cluster (refs #25)

One chart whose values.yaml is a near-literal transcription of
infra/docker-compose.yml, rendered by three generic templates (Deployment, Job,
Service) over a `workloads` map — so the two stacks can be diffed by eye instead
of by archaeology, and adding a service is a values edit.

Platform-forced deviations, each commented where it appears:
- `args`, never `command`: compose replaces the image CMD, Kubernetes replaces the
  ENTRYPOINT. The chart fails to render on `command`, because the symptom (postgres
  refusing to run as root, Keycloak exec-ing `start-dev`) is nothing like the cause.
- The four Django services apply their own setup_configuration in the web pod
  rather than in a separate init Job: both scripts migrate, and without compose's
  depends_on they race the same database.
- OpenZaak and Objecten are addressed by service FQDN, because Django rejects a
  single-label host in a URL — the reason compose passes container IPs around.
- NodePorts, no ingress; databases are emptyDir until persistence.storageClass is
  set, so the stack comes up on a cluster with no CSI driver.

The upstream config inputs stay in the repo and become ConfigMaps via
infra/helm/seed-configmaps.sh — the Kubernetes sibling of infra/seed-config.sh —
so the compose stack and the chart cannot fork. infra/helm/registry.yaml runs an
in-cluster registry because Talos cannot side-load an image and a laptop-side one
needs a root-level firewall change.

infra/helm/big-reference/templates/services.yaml

@@ -0,0 +1,35 @@
+{{- /*
+Service names are the compose service names, verbatim: the portals' Caddy
+proxies to http://bff:8080 and the upstream setup_configuration files name
+http://openzaak:8000 / http://nrc-web:8000, so in-cluster DNS has to answer to
+exactly those names. Do not rename a workload without checking both.
+
+.Values.nodePorts is the single place a port is published outside the cluster;
+a workload listed there gets a NodePort on its first (only) port.
+*/ -}}
+{{- range $name, $w := .Values.workloads }}
+{{- if and (ne $w.enabled false) $w.ports }}
+{{- $nodePort := index $.Values.nodePorts $name }}
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: {{ $name }}
+  labels:
+{{- include "big.labels" (dict "root" $ "name" $name) | nindent 4 }}
+spec:
+  type: {{ if $nodePort }}NodePort{{ else }}ClusterIP{{ end }}
+  selector:
+    app.kubernetes.io/name: {{ $name }}
+    app.kubernetes.io/instance: {{ $.Release.Name }}
+  ports:
+  {{- range $i, $p := $w.ports }}
+    - name: {{ $p.name }}
+      port: {{ $p.port }}
+      targetPort: {{ $p.targetPort | default $p.port }}
+      {{- if and $nodePort (eq $i 0) }}
+      nodePort: {{ $nodePort }}
+      {{- end }}
+  {{- end }}
+{{- end }}
+{{- end }}