feat(app): server-side duplicate pre-check at enqueue moment

Task 2 of docs/plans/2026-09-28-上传查重前置到入队前-实施计划.md:
surface server-side duplicate history at the "click upload" moment,
merged with the local check into the EXISTING dialog.

- ApiClient.checkUploadDuplicates: POST /v1/uploads/duplicates/check
  (commit 805f77b endpoint); 500/chunk sequential requests; chunk-local
  index translated to global; empty list short-circuits (no request);
  errors normalized to AppException via _send.
- enqueue.checkDuplicatesUnified: local TaskStore history ∪ server
  history; server hits map back to filename via global index into the
  stat'd picks; merged, de-duplicated (filename+bytes) list; server
  failure (network/permission/any) silently degrades to local-only —
  init 409 remains the backstop.
- capture_page + album_detail_page _pickFromGallery now call the
  unified entry; dialog/skip/force logic untouched.

Tests: api_client chunking (1200 → 500/500/200) + index translation +
merge + 404 normalization + empty no-request; page-level three states
(hit/no-hit/network failure) incl. fresh-install server-only-hit dialog
on both pages; pre-existing Task 8 dialog tests keep passing via the
degrade path.

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

apps/mobile/lib/core/api_client.dart

@@ -25,6 +25,17 @@ import 'error_codes.dart';
 import 'models/cursor_page.dart';
 import 'session_store.dart';
 
+/// 查重预检请求条目（契约 UploadDuplicateCheckItem；filename 仅元数据——
+/// 服务端查重谓词只看 bytes，与 initUpload ③.5 逐字一致）。
+typedef UploadDuplicateCheckItem = ({String filename, int bytes});
+
+/// 查重预检命中（契约 UploadDuplicateCheckDuplicate；index 已从分块局部
+/// 下标换算为对入参列表的全局下标）。
+typedef UploadDuplicateHit = ({int index, int bytes, DateTime uploadedAt});
+
+/// 单批查重条目上限（契约 items maxItems=500；客户端按此分块顺序调用）。
+const int kUploadDuplicateCheckChunkSize = 500;
+
 class ApiClient {
   /// [dio] 可注入测试替身（自定义 HttpClientAdapter）；[baseUrl] 缺省
   /// 取 [ApiConfig.baseUrl]（--dart-define API_BASE 覆盖）。
@@ -568,6 +579,53 @@ class ApiClient {
     limit: limit,
   );
 
+  /// operationId `checkUploadDuplicates`
+  /// （POST /uploads/duplicates/check，批量只读预检）。
+  ///
+  /// 按 [kUploadDuplicateCheckChunkSize] 顺序分块调用（契约单批上限 500；
+  /// 块间串行防瞬时并发放大）；各块命中的分块局部下标换算为对 [items] 的
+  /// 全局下标后合并返回。空列表直接返回空（契约 items minItems=1，不发
+  /// 请求）。错误经 [_send] 归一为 [AppException]（404/403/400/401 见契约）。
+  Future<List<UploadDuplicateHit>> checkUploadDuplicates(
+    String albumId,
+    List<UploadDuplicateCheckItem> items,
+  ) async {
+    if (items.isEmpty) return const <UploadDuplicateHit>[];
+    final hits = <UploadDuplicateHit>[];
+    for (
+      var start = 0;
+      start < items.length;
+      start += kUploadDuplicateCheckChunkSize
+    ) {
+      final chunk = items
+          .skip(start)
+          .take(kUploadDuplicateCheckChunkSize)
+          .toList();
+      final resp = await _send(
+        () => _dio.post<Map<String, dynamic>>(
+          '/v1/uploads/duplicates/check',
+          data: {
+            'albumId': albumId,
+            'items': [
+              for (final item in chunk)
+                {'filename': item.filename, 'bytes': item.bytes},
+            ],
+          },
+        ),
+      );
+      final duplicates = (resp.data!['duplicates'] as List<dynamic>)
+          .cast<Map<String, dynamic>>();
+      for (final d in duplicates) {
+        hits.add((
+          index: start + (d['index'] as int),
+          bytes: d['bytes'] as int,
+          uploadedAt: DateTime.parse(d['uploadedAt'] as String),
+        ));
+      }
+    }
+    return hits;
+  }
+
   // ── photos（契约 tag: photos）──
 
   /// operationId `listAlbumPhotos`