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.

dev/token-exchange-broker-externalsecret.yaml

@@ -0,0 +1,17 @@
+apiVersion: external-secrets.io/v1
+kind: ExternalSecret
+metadata:
+  name: payments-token-exchange-broker
+  namespace: payments
+spec:
+  refreshInterval: 1h
+  secretStoreRef:
+    kind: ClusterSecretStore
+    name: vault-kong-v2
+  target:
+    name: payments-token-exchange-broker
+  data:
+    - secretKey: TOKEN_EXCHANGE_CLIENT_SECRET
+      remoteRef:
+        key: kong-v2/payments-api/token-exchange-broker
+        property: client_secret