feat: Enhance models and README with additional features and improvements

- Updated README to include KodBox in project vision.
- Added model descriptions for clarity in Download, File, Folder, Group, Node, Order, Policy, Redeem, Report, Request, Response, Setting, Share, SourceLink, StoragePack, Tag, Task, User, and WebDAV.
- Changed optional fields from Optional[...] to the new union type syntax (e.g., str | None).
- Improved foreign key references in models for consistency.
- Refactored relationships in models to use singular forms where appropriate.
- Updated login service to reflect changes in request model types.

models/policy.py

@@ -1,30 +1,29 @@
 
 from typing import Optional, List, TYPE_CHECKING
-from sqlmodel import Field, Relationship, text, Column, func, DateTime
+from sqlmodel import Field, Relationship, text
 from .base import TableBase
-from datetime import datetime
 
 if TYPE_CHECKING:
     from .file import File
     from .folder import Folder
 
 class Policy(TableBase, table=True):
-    __tablename__ = 'policies'
+    """存储策略模型"""
 
     name: str = Field(max_length=255, unique=True, description="策略名称")
     type: str = Field(max_length=255, description="存储类型 (e.g. 'local', 's3')")
-    server: Optional[str] = Field(default=None, max_length=255, description="服务器地址（本地策略为路径）")
-    bucket_name: Optional[str] = Field(default=None, max_length=255, description="存储桶名称")
+    server: str | None = Field(default=None, max_length=255, description="服务器地址（本地策略为路径）")
+    bucket_name: str | None = Field(default=None, max_length=255, description="存储桶名称")
     is_private: bool = Field(default=True, sa_column_kwargs={"server_default": text("true")}, description="是否为私有空间")
-    base_url: Optional[str] = Field(default=None, max_length=255, description="访问文件的基础URL")
-    access_key: Optional[str] = Field(default=None, description="Access Key")
-    secret_key: Optional[str] = Field(default=None, description="Secret Key")
+    base_url: str | None = Field(default=None, max_length=255, description="访问文件的基础URL")
+    access_key: str | None = Field(default=None, description="Access Key")
+    secret_key: str | None = Field(default=None, description="Secret Key")
     max_size: int = Field(default=0, sa_column_kwargs={"server_default": "0"}, description="允许上传的最大文件尺寸（字节）")
     auto_rename: bool = Field(default=False, sa_column_kwargs={"server_default": text("false")}, description="是否自动重命名")
-    dir_name_rule: Optional[str] = Field(default=None, max_length=255, description="目录命名规则")
-    file_name_rule: Optional[str] = Field(default=None, max_length=255, description="文件命名规则")
+    dir_name_rule: str | None = Field(default=None, max_length=255, description="目录命名规则")
+    file_name_rule: str | None = Field(default=None, max_length=255, description="文件命名规则")
     is_origin_link_enable: bool = Field(default=False, sa_column_kwargs={"server_default": text("false")}, description="是否开启源链接访问")
-    options: Optional[str] = Field(default=None, description="其他选项 (JSON格式)")
+    options: str | None = Field(default=None, description="其他选项 (JSON格式)")
     # options 示例: {"token":"","file_type":null,"mimetype":"","od_redirect":"http://127.0.0.1:8000/...","chunk_size":52428800,"s3_path_style":false}
     
     # 关系