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/_helpers.tpl

@@ -0,0 +1,142 @@
+{{/*
+One pod spec for every workload, Deployment and Job alike. The chart is
+values-driven on purpose: `.Values.workloads` is a near-literal transcription of
+infra/docker-compose.yml, so the two stacks can be diffed by eye instead of by
+archaeology. Adding a service is a values edit, not a template edit.
+
+Called as: include "big.podspec" (dict "root" $ "name" $name "w" $w)
+*/}}
+{{- define "big.podspec" -}}
+{{- $root := .root -}}
+{{- $name := .name -}}
+{{- $w := .w -}}
+{{- with $root.Values.imagePullSecrets }}
+imagePullSecrets:
+{{- toYaml . | nindent 2 }}
+{{- end }}
+{{- with $w.waitFor }}
+initContainers:
+  - name: wait-for-deps
+    image: {{ $root.Values.images.busybox }}
+    command:
+      - sh
+      - -c
+      - |
+        for t in {{ join " " . }}; do
+          echo "waiting for $t"
+          until nc -z "${t%:*}" "${t#*:}"; do sleep 2; done
+        done
+{{- end }}
+containers:
+  - name: {{ $name }}
+    image: {{ include "big.image" (dict "root" $root "name" $name "w" $w) }}
+    # Only this repo's images get the configured policy: their `dev` tag is mutable.
+    # Upstream tags are pinned, so IfNotPresent keeps them out of pod-template diffs —
+    # which matters because a changed template makes a Job unpatchable (immutable).
+    imagePullPolicy: {{ if $w.own }}{{ $root.Values.images.pullPolicy }}{{ else }}IfNotPresent{{ end }}
+    {{- if $w.command }}
+    {{- fail (printf "workload %s: use `args`, not `command` — compose's `command:` replaces CMD, but Kubernetes' `command:` replaces the image ENTRYPOINT (postgres would run as root, keycloak would exec `start-dev`)" $name) }}
+    {{- end }}
+    {{- with $w.args }}
+    args:
+{{- toYaml . | nindent 6 }}
+    {{- end }}
+    {{- with $w.envFrom }}
+    envFrom:
+    {{- range . }}
+      - configMapRef:
+          # optional: an env group whose feature is disabled (e.g. otel) simply
+          # isn't rendered, and the pod must still start.
+          name: {{ printf "%s-env" . }}
+          optional: true
+    {{- end }}
+    {{- end }}
+    {{- with $w.env }}
+    env:
+{{- include "big.env" (list $root .) | nindent 6 }}
+    {{- end }}
+    {{- with $w.ports }}
+    ports:
+    {{- range . }}
+      - name: {{ .name }}
+        containerPort: {{ .targetPort | default .port }}
+    {{- end }}
+    {{- end }}
+    {{- with $w.probe }}
+    readinessProbe:
+{{- toYaml . | nindent 6 }}
+    {{- end }}
+    {{- with $w.resources }}
+    resources:
+{{- toYaml . | nindent 6 }}
+    {{- end }}
+    {{- if or $w.files $w.data }}
+    volumeMounts:
+    {{- range $w.files }}
+      - name: {{ .configMap }}
+        mountPath: {{ .mountPath }}
+        {{- with .subPath }}
+        subPath: {{ . }}
+        {{- end }}
+        readOnly: true
+    {{- end }}
+    {{- with $w.data }}
+      - name: data
+        mountPath: {{ .mountPath }}
+    {{- end }}
+    {{- end }}
+{{- if or $w.files $w.data }}
+volumes:
+{{- range $w.files }}
+  - name: {{ .configMap }}
+    configMap:
+      name: {{ .configMap }}
+      {{- with .defaultMode }}
+      defaultMode: {{ . }}
+      {{- end }}
+{{- end }}
+{{- with $w.data }}
+  - name: data
+    {{- if $root.Values.persistence.storageClass }}
+    persistentVolumeClaim:
+      claimName: {{ $name }}-data
+    {{- else }}
+    # No StorageClass configured: the databases are emptyDir, so the stack needs
+    # no CSI driver to come up. Data then lives as long as the pod does — see
+    # docs/runbooks/kubernetes-talos.md for switching on local-path.
+    emptyDir: {}
+    {{- end }}
+{{- end }}
+{{- end }}
+{{- end -}}
+
+{{/* Image ref: `own: true` workloads are built from this repo, everything else is upstream. */}}
+{{- define "big.image" -}}
+{{- $root := .root -}}
+{{- $w := .w -}}
+{{- if $w.own -}}
+{{- $ref := printf "%s/%s:%s" $root.Values.images.repositoryPrefix .name $root.Values.images.tag -}}
+{{- with $root.Values.images.registry }}{{ printf "%s/%s" . $ref }}{{ else }}{{ $ref }}{{ end }}
+{{- else -}}
+{{- $w.image -}}
+{{- end -}}
+{{- end -}}
+
+{{/*
+Env list from a map. Every value is run through `tpl`, so values.yaml can name
+cluster-internal hosts ({{ .Release.Namespace }}) and the node address
+({{ .Values.host }}) without the chart hard-coding either.
+*/}}
+{{- define "big.env" -}}
+{{- $root := index . 0 -}}
+{{- range $k, $v := index . 1 }}
+- name: {{ $k }}
+  value: {{ tpl (toString $v) $root | quote }}
+{{- end }}
+{{- end -}}
+
+{{- define "big.labels" -}}
+app.kubernetes.io/name: {{ .name }}
+app.kubernetes.io/instance: {{ .root.Release.Name }}
+app.kubernetes.io/managed-by: Helm
+{{- end -}}