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

docs/README.md

@@ -7,3 +7,4 @@ Operational notes for the manifests in this repo.
 | [cnpg-backups.md](cnpg-backups.md) | How CNPG Postgres backups (WAL archiving + nightly base backups) to Ceph RGW are configured. |
 | [cnpg-restore.md](cnpg-restore.md) | Restoring a CNPG cluster: full recovery, point-in-time recovery, cutover, and gotchas. |
 | [authentik-rancher-sso.md](authentik-rancher-sso.md) | Manual runtime step to point Rancher's OIDC auth at the canonical `identity.unkin.net` issuer and trust the internal CA. |
+| [gitea-migration.md](gitea-migration.md) | Staged cutover of the git.unkin.net forge from the Puppet VM to the `gitea` namespace. |