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/kongplugin.yaml

@@ -0,0 +1,30 @@
+apiVersion: configuration.konghq.com/v1
+kind: KongPlugin
+metadata:
+  name: ${NAME}-payments-api-openid-connect
+  namespace: ${NAMESPACE}
+plugin: openid-connect
+config:
+  issuer: ${OIDC_ISSUER}
+  audience_required: ${BACKEND_AUDIENCE}
+  auth_methods:
+    - bearer
+  display_errors: false
+  ssl_verify: true
+  consumer_claims:
+    - - sub
+  consumer_by:
+    - custom_id
+    - username
+  consumer_optional: true
+  client_id:
+    - ${TOKEN_EXCHANGE_CLIENT_ID}
+  client_secret:
+    - ${TOKEN_EXCHANGE_CLIENT_SECRET}
+  token_exchange:
+    subject_token_issuers:
+      - issuer: ${OIDC_ISSUER}
+        conditions: {}
+    request:
+      audience: ${BACKEND_AUDIENCE}
+      scopes: ${BACKEND_SCOPES}