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/flux-kustomization.yaml

@@ -0,0 +1,31 @@
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: payments-dev-api
+  namespace: payments
+spec:
+  interval: 10m
+  sourceRef:
+    kind: GitRepository
+    name: payments-api-dev
+  path: ./kong
+  prune: true
+  dependsOn:
+    - name: payments-dev-api-gateway
+      namespace: flux-system
+  postBuild:
+    substitute:
+      NAME: payments-dev
+      NAMESPACE: payments
+      CONTROL_PLANE_REF_NAME: payments-dev-bz6vs
+      BACKEND_HOST: classic-models-api.classic-models.svc.cluster.local
+      OIDC_ISSUER: https://keycloak.sttlab.local/realms/classic-models
+      BACKEND_AUDIENCE: '["classic-models-api"]'
+      BACKEND_SCOPES: '[]'
+      API_RATE_LIMIT_MINUTE: "60"
+      AUDIT_LOG_ENDPOINT: http://audit-log-collector.observability.svc.cluster.local/logs
+      REST_HOSTNAME: payments-dev.sttlab.local
+      TOKEN_EXCHANGE_CLIENT_ID: kong-token-exchange-broker
+    substituteFrom:
+      - kind: Secret
+        name: payments-token-exchange-broker