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/request.py

@@ -13,5 +13,5 @@ class LoginRequest(BaseModel):
     """
     username: str = Field(..., description="用户名或邮箱")
     password: str = Field(..., description="用户密码")
-    captcha: Optional[str] = Field(None, description="验证码")
-    twoFaCode: Optional[str] = Field(None, description="两步验证代码")
+    captcha: str | None = Field(None, description="验证码")
+    twoFaCode: str | None = Field(None, description="两步验证代码")