task-26: 模板 workbook 大行数下的样式、图片和工作表兼容测试

writeWorkbook（模板路径）改为返回实际写入行数，与 streaming 路径一致；
新增 1000/5000 行规模兼容测试：模板表头样式保留、同 URL 图片按 pictureIndex 去重复用、
5 工作表顺序名称保持模板语义、失败行兜底 URL 文本不中断整表。
全量测试 556 通过。

backend-java/src/main/java/com/nanri/aiimage/modules/shopdatacrawl/service/ShopDataCrawlExcelAssemblyService.java

@@ -83,7 +83,7 @@ public class ShopDataCrawlExcelAssemblyService {
         imageEmbedder.prefetch(prefetchBudget().boundedUrls(imageUrls(rowsByCountry)), imageCache);
     }
 
-    public void writeWorkbook(File outputXlsx, List<ShopDataCrawlResultItemVo> items) {
+    public int writeWorkbook(File outputXlsx, List<ShopDataCrawlResultItemVo> items) {
         try (InputStream input = new ClassPathResource(TEMPLATE).getInputStream();
              XSSFWorkbook workbook = new XSSFWorkbook(input);
              FileOutputStream output = new FileOutputStream(outputXlsx)) {
@@ -96,6 +96,7 @@ public class ShopDataCrawlExcelAssemblyService {
                 writeSheet(workbook, workbook.getSheetAt(i), rowsByCountry.get(COUNTRIES.get(i)), imageCache, pictureIndexes);
             }
             workbook.write(output);
+            return rowsByCountry.values().stream().mapToInt(List::size).sum();
         } catch (BusinessException ex) {
             throw ex;
         } catch (Exception ex) {