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/audit-log.yaml

@@ -0,0 +1,17 @@
+apiVersion: configuration.konghq.com/v1
+kind: KongPlugin
+metadata:
+  name: ${NAME}-payments-api-audit-log
+  namespace: ${NAMESPACE}
+plugin: http-log
+# dev only: the raw Authorization (JWT) header is captured in clear text,
+# not acceptable as-is for a non-dev environment. request_body/response_body
+# below read from kong.ctx.shared, populated by audit-log-body-capture.yaml
+# — nil (and silently omitted) if that file isn't applied.
+config:
+  http_endpoint: ${AUDIT_LOG_ENDPOINT}
+  method: POST
+  content_type: application/json
+  custom_fields_by_lua:
+    request_body: "return kong.ctx.shared.audit_request_body"
+    response_body: "return kong.ctx.shared.audit_response_body"