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/token-exchange-broker-externalsecret.yaml

@@ -0,0 +1,17 @@
+apiVersion: external-secrets.io/v1
+kind: ExternalSecret
+metadata:
+  name: customers-token-exchange-broker
+  namespace: customers
+spec:
+  refreshInterval: 1h
+  secretStoreRef:
+    kind: ClusterSecretStore
+    name: vault-kong-v2
+  target:
+    name: customers-token-exchange-broker
+  data:
+    - secretKey: TOKEN_EXCHANGE_CLIENT_SECRET
+      remoteRef:
+        key: kong-v2/customers-api/token-exchange-broker
+        property: client_secret