Build customers-api REST layer (Layer 3), mirroring catalog-api

8 routes over Customer + orders/payments sub-resources, proxying to
classic-models-api. openid-connect with RFC 8693 token exchange to
classic-models-api audience, rate-limiting, proxy-cache, audit-log —
same plugin set and structure as catalog-api's REST surface.

Not deployed: dev/gitrepository.yaml points at a gitea repo that
doesn't exist yet (kong-customers-api.git, 404). No MCP layer yet
either, unlike catalog-api.

kong/rest/service.yaml

@@ -0,0 +1,17 @@
+apiVersion: configuration.konghq.com/v1alpha1
+kind: KongService
+metadata:
+  name: ${NAME}-customers-api
+  namespace: ${NAMESPACE}
+spec:
+  controlPlaneRef:
+    type: konnectNamespacedRef
+    konnectNamespacedRef:
+      name: ${CONTROL_PLANE_REF_NAME}
+  name: customers-api
+  host: ${BACKEND_HOST}
+  port: 8000
+  protocol: http
+  path: /
+  tags:
+    - customers