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/kustomization.yaml

@@ -34,6 +34,9 @@ resources:
   #      (clusters/base/oauth2-proxy.yaml, dependsOn platform-config). See DECISIONS-LOG.
   - cnpg               # CloudNativePG operator (DB backend for Keycloak) — ghcr.io images (not Bitnami)
   - keycloak           # Keycloak Operator (quay.io images) + SOPS admin credential
+  - headlamp           # Kubernetes dashboard — Keycloak native-OIDC login + PER-USER RBAC (ADR-0014).
+                       # HelmRelease + SOPS OIDC credential + default-deny/Cilium policies; its
+                       # HTTPRoute is a Gateway API CR in platform-config (dependsOn platform).
   # ---- Later phases: enabled when their phase opens ----
   # - velero           # Phase 4 (Backup/DR): BLOCKED on OQ-BKP (off-cluster backup target decision)
   # - image-automation # Phase 4 (Automated updates): Flux image-automation + Renovate