Add k8s Gitea deployment (migration target for git.unkin.net) (#309)

Stand up the git.unkin.net 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), so this is genuine multi-replica HA rather than single-replica failover. Serves a temporary git2.k8s.syd1.au.unkin.net host; the git.unkin.net cutover is staged in docs/gitea-migration.md.

- add apps/base/gitea: namespace, CNPG gitea-postgres (2 instances, S3 backup bucket cnpg-gitea, nightly 04:00/30d), pgbouncer pooler, standalone Valkey (session/cache/queue, AOF), VaultAuth + VaultStaticSecrets, Gateway + HTTPRoute
- add apps/overlays/au-syd1/gitea: official Gitea chart 12.6.0 (app 1.26.2, rootless, 2 replicas) via helm-through-kustomize; RWX CephFS repo storage, external CNPG + Valkey, Actions disabled, container registry disabled (moved to artifactapi), Authentik OIDC with auto-register/account-linking; SSH via LoadBalancer VIP 198.18.200.10:2222
- register gitea in the platform ApplicationSet + AppProject
- add docs/gitea-migration.md staged cutover plan (VM Postgres->CNPG dump/restore, DNS in main.unkin.net zone, consumer checklist, rollback)

Depends on: terraform-authentik gitea OIDC app, and terraform-artifactapi ^gitea/ dockerhub allowlist (both separate PRs). One-time Vault seeds are listed in the migration doc.

Reviewed-on: #309
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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'