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.

service/user/login.py

@@ -18,9 +18,9 @@ async def Login(LoginRequest: LoginRequest) -> TokenModel | bool | None:
     :param password: 用户密码
     :type password: str
     :param captcha: 验证码
-    :type captcha: Optional[str]
+    :type captcha: str | None
     :param twoFaCode: 两步验证代码
-    :type twoFaCode: Optional[str]
+    :type twoFaCode: str | None
 
     :return: TokenModel 对象或状态码或 None
     :rtype: TokenModel | int | None