Add k8s Gitea deployment (HA-shaped, migration target for git.unkin.net)

Stand up the forge on k8s to replace the Puppet VM. Deployed HA-shaped to match
what the VM already runs (multi-replica on shared storage + external DB/cache):
official Gitea chart 12.6.0 (app 1.26.2) at 2 replicas on RWX CephFS, CNPG
Postgres with S3 backup, standalone Valkey for cache/session/queue, Authentik
OIDC, Actions disabled and the container registry moved to artifactapi. Serves a
temporary git2.k8s.syd1.au.unkin.net host; cutover is staged (see the doc).

- add apps/base/gitea (namespace, CNPG cluster+backup+pooler, Valkey, VaultAuth,
  VaultStaticSecrets, Gateway, HTTPRoute)
- add apps/overlays/au-syd1/gitea (chart 12.6.0 via helm-through-kustomize + values,
  drop the chart test Pod)
- register gitea in the platform ApplicationSet and AppProject
- add docs/gitea-migration.md staged cutover plan

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

argocd/applicationsets/platform.yaml

@@ -24,6 +24,7 @@ spec:
       - path: apps/overlays/*/elastic-system
       - path: apps/overlays/*/encapi
       - path: apps/overlays/*/externaldns
+      - path: apps/overlays/*/gitea
       - path: apps/overlays/*/grafana-system
       - path: apps/overlays/*/inteldeviceplugins-system
       - path: apps/overlays/*/jfrog

argocd/projects/platform.yaml

@@ -13,6 +13,7 @@ spec:
     - oci://gcr.io/k8s-staging-nfd/charts
     - oci://ghcr.io/woodpecker-ci/helm/woodpecker
     - oci://ghcr.io/netbox-community/netbox-chart/netbox
+    - oci://docker.gitea.com/charts
   destinations:
     - namespace: '*-system'
       server: https://kubernetes.default.svc
@@ -34,6 +35,8 @@ spec:
       server: https://kubernetes.default.svc
     - namespace: 'externaldns'
       server: https://kubernetes.default.svc
+    - namespace: 'gitea'
+      server: https://kubernetes.default.svc
     - namespace: 'jfrog'
       server: https://kubernetes.default.svc
     - namespace: 'kanidm'