fix(history): 请求级下载历史仓储补齐 async_count，修复下载历史接口 500

HistoryQueryService.count_download() 依赖 AsyncDownloadHistoryQueryRepository
协议的 async_count()，但 startup 注入的请求级适配器
SessionDownloadHistoryRepository 只实现了 async_list_by_page()，导致
GET /api/v1/history/download 在写入分页总数响应头时抛 AttributeError 并返回 500，
Web 端下载历史页面无法加载。

为 SessionDownloadHistoryRepository 补齐 async_count()，在请求持有的
AsyncSession 内调用 DownloadHistoryOper.async_count()，Session 类型校验与
async_list_by_page() 保持一致，并补充回归测试。

app/db/adapters/history/download.py

@@ -287,6 +287,12 @@ class SessionDownloadHistoryRepository:
         )
         return [_project_history(record) for record in records]
 
+    async def async_count(self) -> int:
+        """在请求异步 Session 内统计下载历史总数。"""
+        if not isinstance(self._session, AsyncSession):
+            raise RuntimeError("下载历史异步统计需要 AsyncSession")
+        return await DownloadHistoryOper(self._session).async_count()
+
     def stage_delete_history(self, history_id: int) -> None:
         """在请求同步 Session 内暂存下载历史删除。"""
         if not isinstance(self._session, Session):

tests/test_transactional_download_history.py

@@ -205,3 +205,27 @@ def test_session_repository_obeys_caller_transaction(db) -> None:
         repository.stage_delete_history(history.id)
         session.commit()
     assert _repository().get_by_hash("request-history-hash") is None
+
+
+def test_session_repository_counts_history_in_caller_async_session(db) -> None:
+    """请求级 adapter 在调用方 AsyncSession 内统计总数，与分页读取口径一致。"""
+    repository = _repository()
+    baseline_count = asyncio.run(repository.async_count())
+    history_id = repository.add(_history_write(download_hash="request-async-count-hash"))
+
+    async def exercise() -> tuple[int, list[DownloadHistorySnapshot]]:
+        """在同一请求 AsyncSession 内先统计总数再分页读取。"""
+        async with async_session_scope() as session:
+            session_repository = SessionDownloadHistoryRepository(session)
+            total = await session_repository.async_count()
+            records = await session_repository.async_list_by_page(count=10)
+            return total, records
+
+    total, records = asyncio.run(exercise())
+
+    assert total == baseline_count + 1
+    assert any(record.id == history_id for record in records)
+
+    with SessionFactory() as session:
+        with pytest.raises(RuntimeError):
+            asyncio.run(SessionDownloadHistoryRepository(session).async_count())