platform: deploy Headlamp dashboard with Keycloak OIDC + per-user RBAC

Add Headlamp (kubernetes-sigs/headlamp Helm chart) at
headlamp.home.keithjackson.dev with native Keycloak OIDC login and PER-USER
RBAC: Headlamp forwards each user's id_token to the k3s apiserver, so the
existing oidc:* group bindings (platform-admins->cluster-admin, viewers->view)
apply per user. The ServiceAccount is deliberately NOT bound to cluster-admin
(clusterRoleBinding.create=false), since user requests ride the forwarded token.

Key integration nuance: the apiserver is configured oidc-client-id=kubernetes,
so it only accepts tokens whose aud contains `kubernetes`. The new `headlamp`
Keycloak client carries an oidc-audience-mapper (included.client.audience=
kubernetes) so tokens carry aud=[headlamp, kubernetes] -- accepted by both
Headlamp's own verifier and the apiserver, leaving the apiserver config
untouched. A groups mapper emits the `groups` claim the apiserver maps to RBAC.

- platform/headlamp: namespace, HelmRepository/HelmRelease, SOPS OIDC secret
  (externalSecret envFrom), default-deny NetworkPolicy + Cilium ingress
  (gateway)/egress (apiserver+Keycloak) policies, README. Wired into
  platform/kustomization.yaml (platform tier).
- platform/headlamp/httproute.yaml wired into platform-config (Gateway API CR,
  dependsOn platform), modeled on grafana-httproute.
- keycloak-realm-import: add `headlamp` confidential client + groups/audience
  mappers + HEADLAMP_CLIENT_SECRET placeholder. Client also created LIVE via
  kcadm (realm import no-ops on the existing realm).
- keycloak-realm-clients.sops: add headlamp-client-secret (mirrored into
  headlamp-oidc-credentials clientSecret). Both encrypted to the repo age key.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

platform/headlamp/kustomization.yaml

@@ -0,0 +1,18 @@
+---
+# Headlamp — Kubernetes dashboard with Keycloak native-OIDC login + per-user RBAC (ADR-0014).
+# CONTROLLERS half: the HelmRepository + HelmRelease (chart installs the Deployment/Service/SA), the
+# SOPS-encrypted OIDC credential (decrypted by the `platform` Flux Kustomization), and the
+# default-deny + Cilium egress/ingress policies. The HTTPRoute is a Gateway API CR and lives in
+# platform-config (dependsOn platform) so the Gateway CRDs exist first — same split as grafana.
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+namespace: headlamp
+resources:
+  - namespace.yaml
+  - helmrepository.yaml
+  - helmrelease.yaml
+  # OIDC client credential (envFrom'd by the chart via config.oidc.externalSecret) — SOPS-encrypted,
+  # referenced by name (R-IDP-04). The matching value is mirrored in keycloak-realm-clients.
+  - headlamp-oidc-credentials.sops.yaml
+  # default-deny baseline + Cilium ingress(gateway)/egress(apiserver+Keycloak) allows.
+  - networkpolicy.yaml