grafana: deploy Grafana instance, datasource and dashboards

Deploys Grafana in the observability project via the grafana-operator,
mirroring the puppet Grafana but modernised: CNPG for state and Authentik
OIDC for auth. Ports the live datasource + dashboards in as CRs.

Changes (apps/base/grafana):
- CNPG postgres Cluster + rw Pooler (db grafana), plus VaultAuth +
  VaultStaticSecrets pulling postgres/oauth credentials from Vault.
- Grafana CR: postgres backend via the pooler, Authentik generic_oauth
  (client id/secret from the Vault-synced secret, openid/email/profile
  scopes, group->role mapping), root_url grafana.k8s.syd1.au.unkin.net.
- Single GrafanaDatasource (k8s VictoriaMetrics via the operator vmselect
  service); reuses the previous default datasource uid so imported
  dashboards resolve without edits.
- 13 GrafanaDashboards (gzipJson) exported from the current grafana.
- Gateway API (traefik-internal) + HTTPRoute for grafana.k8s.syd1.au.unkin.net.
- Register apps/overlays/*/grafana in the observability ApplicationSet and
  add the grafana namespace to the project destinations.

apps/base/grafana/kustomization.yaml

@@ -0,0 +1,27 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml
+  - cnpg_cluster.yaml
+  - cnpg_pooler.yaml
+  - vaultauth.yaml
+  - vaultstaticsecret.yaml
+  - grafana.yaml
+  - grafanadatasource.yaml
+  - gateway.yaml
+  - httproute.yaml
+  - dashboards/bind9-exporter-dns.yaml
+  - dashboards/ceph-cluster.yaml
+  - dashboards/frr-ospf-route-metrics.yaml
+  - dashboards/gitea.yaml
+  - dashboards/haproxy.yaml
+  - dashboards/media-dashboard.yaml
+  - dashboards/node-exporter-full.yaml
+  - dashboards/nzbget.yaml
+  - dashboards/postgresql-database.yaml
+  - dashboards/puppet-report.yaml
+  - dashboards/victorialogs-cluster.yaml
+  - dashboards/victoriametrics-cluster.yaml
+  - dashboards/victoriametrics-vmagent.yaml