feat: add models for physical files, policies, and user management

- Implement PhysicalFile model to manage physical file references and reference counting.
- Create Policy model with associated options and group links for storage policies.
- Introduce Redeem and Report models for handling redeem codes and reports.
- Add Settings model for site configuration and user settings management.
- Develop Share model for sharing objects with unique codes and associated metadata.
- Implement SourceLink model for managing download links associated with objects.
- Create StoragePack model for managing user storage packages.
- Add Tag model for user-defined tags with manual and automatic types.
- Implement Task model for managing background tasks with status tracking.
- Develop User model with comprehensive user management features including authentication.
- Introduce UserAuthn model for managing WebAuthn credentials.
- Create WebDAV model for managing WebDAV accounts associated with users.

sqlmodels/base/__init__.py

@@ -0,0 +1,12 @@
+"""
+SQLModel 基础模块
+
+包含：
+- SQLModelBase: 所有 SQLModel 类的基类（真正的基类）
+
+注意：
+    TableBase, UUIDTableBase, PolymorphicBaseMixin 已迁移到 sqlmodels.mixin
+    为了避免循环导入，此处不再重新导出它们
+    请直接从 sqlmodels.mixin 导入这些类
+"""
+from .sqlmodel_base import SQLModelBase