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/route.ts

@@ -48,7 +48,7 @@ import type { ImportSingleResult } from "@/types/furni";
 // How many items to import concurrently in a batch. The per-item work is
 // network-bound (asset downloads) and DB-bound, so a modest pool gives a large
 // speedup over the old fully sequential loop without overwhelming the DB.
-const IMPORT_BATCH_CONCURRENCY = 12;
+const IMPORT_BATCH_CONCURRENCY = 3;
 
 async function runWithConcurrency<T, R>(
 	items: T[],
@@ -64,6 +64,7 @@ async function runWithConcurrency<T, R>(
 				const i = cursor++;
 				if (i >= items.length) break;
 				out[i] = await fn(items[i]);
+				await new Promise((r) => setImmediate(r));
 			}
 		},
 	);
@@ -849,6 +850,7 @@ export const PATCH = withAdmin(
 					warning: (err as Error).message,
 				});
 			}
+			await new Promise((r) => setImmediate(r));
 		}
 
 		const succeeded = results.filter((r) => r.ok).length;