perf(metadata): bound bulk re-enrich concurrency

A large batch selection fired hundreds of simultaneous requests (each also
enqueuing a worker job). Run them in batches of BULK_CONCURRENCY.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/features/metadata-editor/MetadataEditorPage.tsx

@@ -65,6 +65,23 @@ function fieldStyle(): React.CSSProperties {
   return { width: '100%' };
 }
 
+// Cap how many bulk metadata ops run at once — a large selection would otherwise
+// fire hundreds of simultaneous requests (each also spins up a worker enqueue).
+const BULK_CONCURRENCY = 6;
+
+async function settleInBatches<T>(
+  items: T[],
+  limit: number,
+  op: (item: T) => Promise<unknown>,
+): Promise<PromiseSettledResult<unknown>[]> {
+  const results: PromiseSettledResult<unknown>[] = [];
+  for (let i = 0; i < items.length; i += limit) {
+    const batch = items.slice(i, i + limit);
+    results.push(...(await Promise.allSettled(batch.map(op))));
+  }
+  return results;
+}
+
 /**
  * `/tracks/:trackId/metadata` — A7 metadata editor: manual edits + AcoustID
  * match picker with a current-vs-proposed diff. `/metadata/batch` operates on a
@@ -723,7 +740,7 @@ function BatchEditor() {
     setBusy(true);
     setResult(null);
     const ids = [...selected];
-    const outcomes = await Promise.allSettled(ids.map(op));
+    const outcomes = await settleInBatches(ids, BULK_CONCURRENCY, op);
     const ok = outcomes.filter((o) => o.status === 'fulfilled').length;
     setResult({ ok, fail: ids.length - ok });
     setBusy(false);