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/mixin/__init__.py

@@ -0,0 +1,62 @@
+"""
+SQLModel Mixin模块
+
+提供各种Mixin类供SQLModel实体使用。
+
+包含：
+- polymorphic: 联表继承工具（create_subclass_id_mixin, AutoPolymorphicIdentityMixin, PolymorphicBaseMixin）
+- optimistic_lock: 乐观锁（OptimisticLockMixin, OptimisticLockError）
+- table: 表基类（TableBaseMixin, UUIDTableBaseMixin）
+- table: 查询参数类（TimeFilterRequest, PaginationRequest, TableViewRequest）
+- relation_preload: 关系预加载（RelationPreloadMixin, requires_relations）
+- jwt/: JWT认证相关（JWTAuthMixin, JWTManager, JWTKey等）- 需要时直接从 .jwt 导入
+- info_response: InfoResponse DTO的id/时间戳Mixin
+
+导入顺序很重要，避免循环导入：
+1. polymorphic（只依赖 SQLModelBase）
+2. optimistic_lock（只依赖 SQLAlchemy）
+3. table（依赖 polymorphic 和 optimistic_lock）
+4. relation_preload（只依赖 SQLModelBase）
+
+注意：jwt 模块不在此处导入，因为 jwt/manager.py 导入 ServerConfig，
+而 ServerConfig 导入本模块，会形成循环。需要 jwt 功能时请直接从 .jwt 导入。
+"""
+# polymorphic 必须先导入
+from .polymorphic import (
+    AutoPolymorphicIdentityMixin,
+    PolymorphicBaseMixin,
+    create_subclass_id_mixin,
+    register_sti_column_properties_for_all_subclasses,
+    register_sti_columns_for_all_subclasses,
+)
+# optimistic_lock 只依赖 SQLAlchemy，必须在 table 之前
+from .optimistic_lock import (
+    OptimisticLockError,
+    OptimisticLockMixin,
+)
+# table 依赖 polymorphic 和 optimistic_lock
+from .table import (
+    ListResponse,
+    PaginationRequest,
+    T,
+    TableBaseMixin,
+    TableViewRequest,
+    TimeFilterRequest,
+    UUIDTableBaseMixin,
+    now,
+    now_date,
+)
+# relation_preload 只依赖 SQLModelBase
+from .relation_preload import (
+    RelationPreloadMixin,
+    requires_relations,
+)
+# jwt 不在此处导入（避免循环：jwt/manager.py → ServerConfig → mixin → jwt）
+# 需要时直接从 sqlmodels.mixin.jwt 导入
+from .info_response import (
+    DatetimeInfoMixin,
+    IntIdDatetimeInfoMixin,
+    IntIdInfoMixin,
+    UUIDIdDatetimeInfoMixin,
+    UUIDIdInfoMixin,
+)