Improved private staff Discord relay flow: StaffChat now controls its own staff channel routing, keeps public relay behaviour untouched, reuses provider formatting, and only prepends the StaffChat prefix.

Allow inbound messages from Discord to server chat. Use of DiscordSRV / Essentials API to stop repeat configuration in StaffChat.

Allow messages in the chat with the prefix to be sent to a dedicated Discord channel within a server via DiscordSRV or Essentials.