docs: add reference to custom user verifier demo server

Add an info callout to the "Secure Auth in Production" guide pointing
readers to the custom-user-verifier demo repo as a working example
they can run locally or hand to a coding agent.

Made-with: Cursor

Author: byrro

app/en/guides/user-facing-agents/secure-auth-production/page.mdx

@@ -47,6 +47,14 @@ When your users authorize a tool, Arcade.dev will redirect the user's browser to
 
 If you need help, join the [Implementing a custom user verifier](https://github.com/ArcadeAI/arcade-ai/discussions/486) GitHub discussion and we'll be happy to assist.
 
+<Callout type="info">
+  For a complete working example, see the [custom user verifier demo
+  server](https://github.com/ArcadeAI/custom-user-verifier). It's a minimal
+  Python Flask app you can run locally to test the verification flow, use as a
+  reference when building your own, or to provide as a sample to your coding
+  agent (e.g. Claude Code, Cursor Agent, etc).
+</Callout>
+
 import { Steps, Tabs, Callout } from "nextra/components";
 
 <Steps>