Restricts Discourse reply-by-email to private messages only. Email replies to public topics are rejected (the sender gets Discourse's standard "reply on the web instead" notice). PM replies post normally.
reply_by_email_enabled is a global Discourse setting with no PM-vs-topic split;
this plugin adds that split by guarding Email::Receiver#process_destination and
raising ReplyNotAllowedError when the resolved topic isn't a private message.
Pairs with the forum-reply-ingest Cloudflare Email Worker (inbound pipe) in the
sweb repo. Saturday Inc internal infra.