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/api/admin/import/clone/batch/route.ts

@@ -67,7 +67,7 @@ export const POST = withAdmin(
 
 		return runSseBatch<BatchItem>({
 			items,
-			concurrency: body.concurrency || 6,
+			concurrency: body.concurrency || 10,
 			signal: request.signal,
 			labelOf: (it) => it.classname,
 			worker: async (it, _index, report) => {

src/app/api/admin/import/clone/sync-all/route.ts

@@ -108,7 +108,7 @@ export const POST = withAdmin(
 
 		return runSseBatch({
 			items: allItems,
-			concurrency: 6,
+			concurrency: 10,
 			signal: request.signal,
 			labelOf: (it) => `${it.sourceName}/${it.classname}`,
 			worker: async (it, _index, report) => {