fix(httpd): update server_test for termMgr arg after rebase

The shutdown endpoint test was authored against the pre-rebase
httpd.New(cfg, log) signature. After rebasing onto main, the terminal
manager (from #50) made termMgr a required third arg. Pass nil — the
test exercises /shutdown, not /mux, so the terminal surface stays off.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/httpd/server_test.go

@@ -100,7 +100,7 @@ func TestServerShutdownEndpoint(t *testing.T) {
 		RunFilePath:     runPath,
 	}
 
-	srv, err := New(cfg, discardLogger())
+	srv, err := New(cfg, discardLogger(), nil)
 	if err != nil {
 		t.Fatalf("New: %v", err)
 	}