fix: reduce import concurrency to prevent 100% CPU during SWF→Nitro conversion

- Default batch concurrency: 3→1 (max 5→3)
- Default batch-regen concurrency: 3→1 (max 5→3)
- Default IMPORT_BATCH_CONCURRENCY: 12→3
- Added setImmediate yields between items to prevent event loop blocking

Fixes gatje and other complex furniture imports consuming 100% CPU and stalling the catalog studio.

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

@@ -39,7 +39,7 @@ export const POST = withAdmin(
 	async (request) => {
 		const body = await request.json();
 		const items: RegenItem[] = body.items || [];
-		const concurrency = Math.min(Math.max(body.concurrency || 3, 1), 5);
+		const concurrency = Math.min(Math.max(body.concurrency || 1, 1), 3);
 
 		if (items.length === 0) return apiError("No items to regenerate", 400);
 
@@ -237,6 +237,7 @@ export const POST = withAdmin(
 					});
 
 					await Promise.allSettled(promises);
+					await new Promise((r) => setImmediate(r));
 				}
 
 				// Batch write FurnitureData.json