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/proxy-cache.yaml

@@ -0,0 +1,16 @@
+apiVersion: configuration.konghq.com/v1
+kind: KongPlugin
+metadata:
+  name: ${NAME}-payments-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