Add reference to custom user verifier demo server

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

@@ -47,6 +47,19 @@ 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 (for example Claude Code, Cursor Agent, etc).
+
+  If you're using a coding agent to help build your verifier, give it the
+  [build a custom user verifier](https://github.com/ArcadeAI/arcade-dev-skills/tree/main/build-custom-user-verifier)
+  skill — it contains the full flow, pseudocode, and common mistakes in an
+  LLM-friendly format.
+</Callout>
+
 import { Steps, Tabs, Callout } from "nextra/components";
 
 <Steps>