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.

dev/gitrepository.yaml

@@ -0,0 +1,12 @@
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: customers-api-dev
+  namespace: customers
+spec:
+  interval: 5m
+  url: ssh://git@gitea.sttlab.eu:2222/stt/kong-customers-api.git
+  secretRef:
+    name: customers-api-git-auth
+  ref:
+    branch: main