deploy encapi to au-syd1 (#230)

## Why
encapi is the new Postgres-backed Puppet ENC that replaces Cobbler (Go API + encapi-cli + terraform provider). It needs to run somewhere reachable by the puppet masters (`encapi-cli classify`) and every node's `enc_direct_facts` fact. Deploy it in k8s alongside artifactapi, exposed at `encapi.k8s.syd1.au.unkin.net`.

## Changes
- add `apps/base/encapi/`: namespace, deployment (`git.unkin.net/unkin/encapi`, port 8000, `/healthz` probes), service, gateway + httproute (`encapi.k8s.syd1.au.unkin.net`, traefik-internal), configmap (DB coordinates), CNPG cluster + pooler (database `encapi`), and VaultAuth + VaultStaticSecrets (`postgres-credentials`, `environment`)
- add `apps/overlays/au-syd1/encapi` overlay referencing the base
- register `apps/overlays/*/encapi` in the platform ApplicationSet so ArgoCD picks it up

## Notes
- Mirrors the artifactapi pattern (VaultAuth role `default`, namespace-scoped VSO paths `kv/kubernetes/namespace/encapi/default/*`).
- Before first sync, seed the Vault KV secrets: `environment` must carry `DBPASS` (matching the CNPG owner password) and `ENCAPI_WRITE_TOKEN`; `postgres-credentials` carries the CNPG owner username/password.
- `kustomize build apps/overlays/au-syd1/encapi` validates clean (11 resources).

---------

Co-authored-by: unkinben <neotheo@gmail.com>
Reviewed-on: #230
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

apps/base/encapi/cnpg_pooler.yaml

@@ -0,0 +1,33 @@
+---
+apiVersion: postgresql.cnpg.io/v1
+kind: Pooler
+metadata:
+  name: postgres-pooler
+  namespace: encapi
+spec:
+  cluster:
+    name: postgres
+  instances: 2
+  pgbouncer:
+    parameters:
+      default_pool_size: "100"
+      max_client_conn: "400"
+    paused: false
+    poolMode: session
+  template:
+    metadata:
+      labels:
+        app: pooler
+    spec:
+      affinity:
+        podAntiAffinity:
+          requiredDuringSchedulingIgnoredDuringExecution:
+            - labelSelector:
+                matchExpressions:
+                  - key: app
+                    operator: In
+                    values:
+                      - pooler
+              topologyKey: kubernetes.io/hostname
+      containers: []
+  type: rw