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/test/pages/capture_page_test.dart

@@ -165,7 +165,11 @@ void main() {
         categories: categories,
       );
 
-  Future<void> pumpCapture(WidgetTester tester, {CaptureSource? cameraSource}) async {
+  Future<void> pumpCapture(
+    WidgetTester tester, {
+    CaptureSource? cameraSource,
+    FakeAdapter? apiAdapter,
+  }) async {
     final effectiveSource = cameraSource ?? source;
     await tester.pumpWidget(
       ProviderScope(
@@ -176,8 +180,12 @@ void main() {
             client: ApiClient(
               sessionStore: session,
               dio: Dio(BaseOptions(baseUrl: 'http://test.local'))
-                ..httpClientAdapter = FakeAdapter(
-                    (_) => throw StateError('连拍页不应发起 API 请求')),
+                ..httpClientAdapter = apiAdapter ??
+                    FakeAdapter(
+                      // 多选入队的查重预检会发 API 请求：默认打桩为失败，
+                      // 驱动「服务器预检失败 → 静默降级本机-only」路径。
+                      (_) => throw StateError('连拍页测试 API 请求一律失败'),
+                    ),
             ),
           ),
           captureSourceFactoryProvider.overrideWith(
@@ -462,6 +470,50 @@ void main() {
     );
   });
 
+  testWidgets('服务器历史查重命中（本机无记录）→ 统一查重弹窗，取消不上传（Task 2）',
+      (tester) async {
+    // 新设备场景：本机 TaskStore 空，服务器预检命中 pick1（bytes 一致）；
+    // pick2 字节数不同不命中——弹窗应精确提示 1 张重复。
+    final adapter = FakeAdapter((o) {
+      if (o.path == '/v1/uploads/duplicates/check' && o.method == 'POST') {
+        return (
+          200,
+          {
+            'duplicates': [
+              {
+                'index': 0,
+                'bytes': kJpgBytes.length,
+                'uploadedAt': '2026-09-25T08:00:00Z',
+              },
+            ],
+          },
+        );
+      }
+      throw StateError('unexpected ${o.method} ${o.path}');
+    });
+    final pick1 = File('${tmp.path}/pick1.jpg')..writeAsBytesSync(kJpgBytes);
+    final pick2 = File('${tmp.path}/pick2.jpg')
+      ..writeAsBytesSync([...kJpgBytes, 0x00]);
+    galleryPaths.addAll([pick1.path, pick2.path]);
+    await pumpCapture(tester, apiAdapter: adapter);
+
+    await pressRealIo(tester, Icons.photo_library_outlined);
+    await tester.pump(const Duration(milliseconds: 300));
+
+    expect(
+      find.text('可能已上传过'),
+      findsOneWidget,
+      reason: '服务器历史命中（新设备场景）也弹查重确认',
+    );
+    expect(find.textContaining('1 张照片可能已上传到该相册'), findsOneWidget);
+    expect(find.text('跳过重复，上传 1 张'), findsOneWidget);
+
+    // 取消：整批不上传（服务器命中同样尊重用户取消）。
+    await tester.tap(find.text('取消'));
+    await tester.pumpAndSettle();
+    expect(await store.listByAccount('alice'), isEmpty);
+  });
+
   testWidgets('退出确认：有待传任务 → 弹确认；保留继续拍 / 确认才退出',
       (tester) async {
     await pumpCapture(tester);