feat(media): add single-server SFU scaffolding and media mode config

Backend/media/sfu/service.ts

@@ -0,0 +1,14 @@
+import { mediaMode } from '../config';
+import { NoopSfuService } from './noop';
+import type { SfuService } from './types';
+
+const createSfuService = (): SfuService | null => {
+  if (mediaMode !== 'single_server_sfu') {
+    return null;
+  }
+
+  return new NoopSfuService();
+};
+
+export const sfuService = createSfuService();
+