Add payments-mcp (Layer 3 MCP surface), single-route pattern from the start

list-payments/get-payment (all 5 roles) and create-payment (admin
only — matches PaymentPermission's actual write-access logic, not
guessed). One KongRoute for GET/POST/DELETE, one ai-mcp-proxy
binding — avoids the duplicate-tool-name bug found and fixed on
customers-mcp (STATUS.md section 18), built correctly here instead
of repeating the two-route mistake.

Keycloak (local): mcp-payments-audience client scope + mcp-payments
resource client, attached to librechat as an optional scope, no
dedicated protocol mappers on the client itself.

dev/flux-kustomization.yaml

@@ -20,6 +20,7 @@ spec:
       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
+      API_AUDIENCE: '["payments-api"]'
       BACKEND_AUDIENCE: '["classic-models-api"]'
       BACKEND_SCOPES: '[]'
       API_RATE_LIMIT_MINUTE: "60"