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/proxy-cache.yaml

@@ -0,0 +1,16 @@
+apiVersion: configuration.konghq.com/v1
+kind: KongPlugin
+metadata:
+  name: ${NAME}-customers-api-proxy-cache
+  namespace: ${NAMESPACE}
+plugin: proxy-cache-advanced
+config:
+  strategy: memory
+  request_method:
+    - GET
+    - HEAD
+  response_code:
+    - 200
+  content_type:
+    - application/json
+  cache_ttl: 60