Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS

Summary The SSE event server bound to 0.0.0.0:5553 on Linux/macOS by default because the platform-dependent host default in engine/flags.go:39-46 set host = "" for non-Windows, and utils.JoinHostPort"", ":5553" resolves to ":5553" — a Go http.Server.Addr of ":5553" listens on every interface. On...