refactor(backend): 存在確認のfindOneByをexistに置き換え (#11224)

* refactor(backend): 存在確認の`findOneBy`を`exist`に置き換え

* cleanup

packages/backend/src/server/api/stream/channels/user-list.ts

@@ -34,11 +34,13 @@ class UserListChannel extends Channel {
 		this.listId = params.listId as string;
 
 		// Check existence and owner
-		const list = await this.userListsRepository.findOneBy({
-			id: this.listId,
-			userId: this.user!.id,
+		const listExist = await this.userListsRepository.exist({
+			where: {
+				id: this.listId,
+				userId: this.user!.id,
+			},
 		});
-		if (!list) return;
+		if (!listExist) return;
 
 		// Subscribe stream
 		this.subscriber.on(`userListStream:${this.listId}`, this.send);