feat: initial commit — unarr CLI

Search, inspect, stream, and download torrents from the terminal.
Replaces the entire *arr stack with a single binary.

internal/engine/usenet.go

@@ -0,0 +1,26 @@
+package engine
+
+import (
+	"context"
+	"fmt"
+)
+
+// UsenetDownloader downloads via Usenet/NZB protocol.
+// Currently a stub — not implemented.
+type UsenetDownloader struct{}
+
+func NewUsenetDownloader() *UsenetDownloader { return &UsenetDownloader{} }
+
+func (u *UsenetDownloader) Method() DownloadMethod { return MethodUsenet }
+
+func (u *UsenetDownloader) Available(_ context.Context, _ *Task) (bool, error) {
+	return false, nil // always unavailable until implemented
+}
+
+func (u *UsenetDownloader) Download(_ context.Context, _ *Task, _ string, _ chan<- Progress) (*Result, error) {
+	return nil, fmt.Errorf("usenet download not implemented yet (coming in a future release)")
+}
+
+func (u *UsenetDownloader) Pause(_ string) error             { return nil }
+func (u *UsenetDownloader) Cancel(_ string) error            { return nil }
+func (u *UsenetDownloader) Shutdown(_ context.Context) error { return nil }