Add option to require users to pay to publish a comment

https://stacker.news/items/74045

basically the option to put the comments feature behind the paywall. The user would pay an invoice and once the invoice is paid a comment can be posted.

Start with the linked Stacker News request and trace the plugin's existing comment and payment flows. No file, test, or entry point is named in the issue, so first identify where comments are published and invoices are handled; done means documenting and implementing a clear paywall option that permits posting only after payment.

Written by the indexing model from the issue text.