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/node.py

@@ -1,23 +1,22 @@
 
-from typing import Optional, TYPE_CHECKING
-from sqlmodel import Field, Relationship, text, Column, func, DateTime
+from typing import TYPE_CHECKING
+from sqlmodel import Field, Relationship, text
 from .base import TableBase
-from datetime import datetime
 
 if TYPE_CHECKING:
     from .download import Download
 
 class Node(TableBase, table=True):
-    __tablename__ = 'nodes'
+    """节点模型"""
     
     status: int = Field(default=0, sa_column_kwargs={"server_default": "0"}, description="节点状态: 0=正常, 1=离线")
     name: str = Field(max_length=255, unique=True, description="节点名称")
     type: int = Field(default=0, sa_column_kwargs={"server_default": "0"}, description="节点类型")
     server: str = Field(max_length=255, description="节点地址（IP或域名）")
-    slave_key: Optional[str] = Field(default=None, description="从机通讯密钥")
-    master_key: Optional[str] = Field(default=None, description="主机通讯密钥")
+    slave_key: str | None = Field(default=None, description="从机通讯密钥")
+    master_key: str | None = Field(default=None, description="主机通讯密钥")
     aria2_enabled: bool = Field(default=False, sa_column_kwargs={"server_default": text("false")}, description="是否启用Aria2")
-    aria2_options: Optional[str] = Field(default=None, description="Aria2配置 (JSON格式)")
+    aria2_options: str | None = Field(default=None, description="Aria2配置 (JSON格式)")
     rank: int = Field(default=0, sa_column_kwargs={"server_default": "0"}, description="节点排序权重")
 
     # 关系