perf(import): adjustable concurrency and skip redundant SWF downloads

Raise batch concurrency (furni 3->12, clone 10->12) with a Speed control next to Translate. Skip the SWF download when a .nitro bundle already exists on disk (color variants share the base nitro), and stop flagging that as a failed download.

src/app/api/admin/import/furni/batch/route.ts

@@ -43,7 +43,7 @@ export const POST = withAdmin(
 	async (request, ctx) => {
 		const body = await request.json();
 		const rawItems: BatchItem[] = body.items || [];
-		const concurrency = Math.min(Math.max(body.concurrency || 1, 1), 3);
+		const concurrency = Math.min(Math.max(body.concurrency || 3, 1), 12);
 		const sourceId: string | undefined = body.sourceId;
 
 		if (rawItems.length === 0) return apiError("No items to import", 400);