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

6 routes over Payment, composite key (customernumber + checknumber),
proxying to classic-models-api. openid-connect with RFC 8693 token
exchange to classic-models-api audience, rate-limiting, proxy-cache,
audit-log — same structure as customers-api's REST layer.

Keycloak (local): payments-api-audience client scope + payments-api
resource client added, attached to kong-token-exchange-broker's
default scopes. No dedicated protocol mappers on payments-api
itself, same practice as customers-api/mcp-customers.

No MCP layer — same as customers-api, not needed yet.

kong/rest/service.yaml

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