功能(文档与桌面)：完善文件预览下载与客户端构建基线

- 保存文档版本原始文件名，规范下载响应并持久化上传目录\n- 增加 PDF.js 预览、桌面保存打开流程及统一错误反馈\n- 统一 Node.js 22.13 构建基线并收紧临时文件权限门禁\n- 补充迁移、单元测试、发布检查与运维文档

backend/alembic/versions/20260713_02_add_document_version_original_filename.py

@@ -0,0 +1,25 @@
+"""Store original filenames for document versions.
+
+Revision ID: 20260713_02
+Revises: 20260713_01
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+
+revision = "20260713_02"
+down_revision = "20260713_01"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "document_versions",
+        sa.Column("original_filename", sa.String(length=255), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("document_versions", "original_filename")