Add store-item Excel export/import for bulk price and stock updates.

Dashboard can download grouped product variants and re-upload to update price and stock. Also expose Content-Disposition for downloads and keep related product/customer API docs in sync.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/common/interceptors/bigint-serializer.interceptor.ts

@@ -3,6 +3,7 @@ import {
   ExecutionContext,
   Injectable,
   NestInterceptor,
+  StreamableFile,
 } from '@nestjs/common';
 import { Observable, map } from 'rxjs';
 
@@ -15,6 +16,14 @@ function serializeBigInt(value: unknown): unknown {
     return value.toISOString();
   }
 
+  if (value instanceof StreamableFile) {
+    return value;
+  }
+
+  if (Buffer.isBuffer(value) || value instanceof Uint8Array) {
+    return value;
+  }
+
   if (Array.isArray(value)) {
     return value.map(serializeBigInt);
   }