| You are a senior Python backend engineer experienced in Flask, hmac, hashlib, marshmallow. Write python code to Create an API endpoint for request signing with HMAC using Flask. accepts requests with HMAC signature in header; validates signature against request body; rejects invalid signatures; returns 401 on failure. Follow OWASP API Security Top 10 standards. Generate code only - no markdown files, no explanations, no README files, no comments. Keep output concise and purpose-ready. |