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/rate-limiting.yaml

@@ -0,0 +1,14 @@
+apiVersion: configuration.konghq.com/v1
+kind: KongPlugin
+metadata:
+  name: ${NAME}-payments-api-rate-limiting
+  namespace: ${NAMESPACE}
+plugin: rate-limiting-advanced
+config:
+  limit:
+    - ${API_RATE_LIMIT_MINUTE}
+  window_size:
+    - 60
+  identifier: consumer
+  sync_rate: -1
+  strategy: local