perf(C6): 去重总数据列表顺序翻页改 keyset（前后端契约一起改）

背景：列表页 `ORDER BY id DESC LIMIT offset,size`，带筛选且选择性低时每页都要
对命中集做一次 filesort；深翻页 offset 也白扫索引。

改法（保持跳页/回退/改每页的原有行为）：
- 后端：page 接口新增可选 `last_id` 游标 —— 传了就 `id < lastId` + `LIMIT size`（无 offset），
  响应新增 `nextLastId`（本页最后一行 id）；不传仍是原 OFFSET 分页
- 管理前端：只有"下一页"用游标（上一页响应带回），跳页/改每页/筛选清空游标走 OFFSET
- 测试：新增 2 个后端契约测试（keyset 无 offset + nextLastId；无游标保持 LIMIT 30,15）

验证：mvn test 2897 全绿；admin-frontend-vue vue-tsc 通过 + 1619 测试全绿；
已部署 JAR 2b9ab774c60c3f5d802c9a2cee549008（双节点 health=200）与 admin-vue-20260914-112055。

backend-java/src/main/java/com/nanri/aiimage/modules/dedupe/controller/DedupeTotalDataController.java

@@ -88,10 +88,12 @@ public class DedupeTotalDataController {
             @RequestParam(name = "end_date", required = false) @DateTimeFormat(iso = DateTimeFormat.ISO.DATE) LocalDate endDate,
             @Parameter(description = "分组 ID") @RequestParam(name = "group_id", required = false) Long groupId,
             @Parameter(description = "国家代码（如 DE、UK）") @RequestParam(name = "country", required = false) String country,
+            @Parameter(description = "顺序翻页游标（上一页返回的 nextLastId；传了就忽略 page 偏移）")
+            @RequestParam(name = "last_id", required = false) Long lastId,
             HttpServletRequest request) {
         RequestOperator operator = requireDedupeTotalDataAccess(request);
         return ApiResponse.success(dedupeTotalDataService.page(
-                page, pageSize, keyword, username, startDate, endDate, groupId, country, operator.id()));
+                page, pageSize, keyword, username, startDate, endDate, groupId, country, lastId, operator.id()));
     }
 
     @GetMapping("/export")

backend-java/src/main/java/com/nanri/aiimage/modules/dedupe/model/vo/DedupeTotalDataPageVo.java

@@ -20,4 +20,9 @@ public class DedupeTotalDataPageVo {
 
     @Schema(description = "每页数量")
     private Long pageSize;
+    /**
+     * 顺序翻页游标（2026-09 审查 C6）：本页最后一行的 id；下一页把它回传为 last_id
+     * 即可走 keyset（索引顺序扫描 + LIMIT），避免深分页的 offset 代价。跳页仍用 page。
+     */
+    private Long nextLastId;
 }

backend-java/src/main/java/com/nanri/aiimage/modules/dedupe/service/DedupeTotalDataService.java

@@ -156,6 +156,16 @@ public class DedupeTotalDataService {
 
     public DedupeTotalDataPageVo page(long page, long pageSize, String keyword, String username,
                                       LocalDate startDate, LocalDate endDate, Long groupId, String country, Long operatorId) {
+        return page(page, pageSize, keyword, username, startDate, endDate, groupId, country, null, operatorId);
+    }
+
+    /**
+     * @param lastId keyset 游标（上一页返回的 nextLastId）；非空时忽略 page 偏移，
+     *               直接按 id 倒序取"比它小"的一页，避免深分页 offset 扫索引的代价
+     */
+    public DedupeTotalDataPageVo page(long page, long pageSize, String keyword, String username,
+                                      LocalDate startDate, LocalDate endDate, Long groupId, String country,
+                                      Long lastId, Long operatorId) {
         if (startDate != null && endDate != null && startDate.isAfter(endDate)) {
             throw new BusinessException("开始日期不能晚于结束日期");
         }
@@ -181,8 +191,14 @@ public class DedupeTotalDataService {
                 .orderByDesc(DedupeTotalDataEntity::getId);
         applyGroupScope(query, scope, groupId);
         Long total = dedupeTotalDataMapper.selectCount(query);
+        boolean keyset = lastId != null && lastId > 0;
+        if (keyset) {
+            query.lt(DedupeTotalDataEntity::getId, lastId);
+        }
         List<DedupeTotalDataEntity> rows = dedupeTotalDataMapper.selectList(
-                query.last("LIMIT " + ((safePage - 1) * safePageSize) + ", " + safePageSize));
+                keyset
+                        ? query.last("LIMIT " + safePageSize)
+                        : query.last("LIMIT " + ((safePage - 1) * safePageSize) + ", " + safePageSize));
         Map<Long, String> groupNames = loadGroupNames(rows);
         List<DedupeTotalDataItemVo> items = rows.stream()
                 .map(row -> toItemVo(row, row.getGroupId() == null
@@ -194,6 +210,8 @@ public class DedupeTotalDataService {
         vo.setTotal(total);
         vo.setPage(safePage);
         vo.setPageSize(safePageSize);
+        // 顺序翻页游标：本页最后一行的 id（空页给 null，前端据此停止换页）
+        vo.setNextLastId(items.isEmpty() ? null : items.get(items.size() - 1).getId());
         return vo;
     }