Meta App for Automated DMs

I want to take a Meta developer app all the way into production so my backend can seamlessly watch for new comments and, whenever a keyword appears, drop the commenter an automatic direct message across Instagram, Facebook Messenger and WhatsApp.

Here is what I need built and handed over:
• A single Meta app configured for Instagram, Messenger and WhatsApp messaging, fully approved for production status with the required permissions.
• Webhooks set up so I receive real-time notifications as soon as a comment is posted.
• An authentication layer that relies on a simple API Key; once authorised, my server must be able to hit the Graph API endpoints and send the DM.
• Lightweight sample code (any mainstream language is fine) showing how to:
 1. subscribe to comment events,
 2. parse the comment for the target keyword,
 3. send the personalised message back to that user on the same platform.
• A short read-me that documents setup steps, endpoints and any callback URLs I need to register.

I will test by posting comments containing the key phrase on each platform and will sign off once the matching DM lands in under a few seconds every time.