feat(assets): 批量删除接口 + 分页性能优化

- 新增 POST /assets/batch-delete 批量删除素材接口
  - 仓储层: abstract/SQLAlchemy/InMemory 均实现 batch_delete()
  - API层: 逐项校验项目权限后批量删除
  - Schema: BatchDeleteRequest/BatchDeleteResponse
- GET /assets 分页优化: 无 keyword/gender/style 过滤时走 DB 级 skip/limit
- 素材上传 duration 字段已全链路支持（domain/schema/ORM/repository）
- 新增 4 个批量删除单元测试，全量 1030 测试通过

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/adapters/sqlalchemy_impl/asset_repository.py

@@ -120,6 +120,14 @@ class SQLAlchemyAssetRepository:
             return True
         return False
 
+    def batch_delete(self, asset_ids: list[str]) -> int:
+        """批量删除素材，返回实际删除数量。"""
+        if not asset_ids:
+            return 0
+        count = self.session.query(AssetModel).filter(AssetModel.id.in_(asset_ids)).delete(synchronize_session=False)
+        self.session.commit()
+        return count
+
     def count_by_project(self, project_id: str) -> int:
         return self.session.query(AssetModel).filter(AssetModel.project_id == project_id).count()