perf(import): parallelize ID allocation and raise clone concurrency to 10

Replace the serialized SELECT MAX + INSERT id-allocation chains for
items_base and catalog_items with a lazy-seeded in-process counter so
concurrent clone workers no longer queue on a global lock per item.
Re-seeds after 60s idle to avoid colliding with externally added rows.
Raise the clone import concurrency default from 6 to the batch cap of 10.

src/app/admin/import/clone/import-clone-client.tsx

@@ -454,7 +454,7 @@ function FurniGrid({ source }: FurniGridProps) {
 				{
 					sourceId: source.id,
 					classnames: toClone.map((it) => it.classname),
-					concurrency: 6,
+					concurrency: 10,
 				},
 				(classname) => {
 					done++;
@@ -545,7 +545,7 @@ function FurniGrid({ source }: FurniGridProps) {
 				const chunk = names.slice(i, i + CHUNK);
 				await runSseImport(
 					"/api/admin/import/clone/batch",
-					{ sourceId: source.id, classnames: chunk, concurrency: 6 },
+					{ sourceId: source.id, classnames: chunk, concurrency: 10 },
 					(classname) => {
 						done++;
 						markDone(classname);