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/user.py

@@ -15,7 +15,7 @@ from .task import Task
 from .webdav import WebDAV
 
 class User(TableBase, table=True):
-    __tablename__ = 'users'
+    """用户模型"""
 
     email: str = Field(max_length=100, unique=True, index=True)
     """用户邮箱，唯一"""
@@ -25,44 +25,56 @@ class User(TableBase, table=True):
     
     nick: str | None = Field(default=None, max_length=50)
     """用户昵称"""
+
     password: str = Field(max_length=255)
     """用户密码（加密后）"""
-    status: bool | None = Field(default=None, sa_column_kwargs={"server_default": "0"})
-    """用户状态: True=正常, None=未激活, False=封禁"""
+
+    status: bool = Field(default=True, sa_column_kwargs={"server_default": "true"})
+    """用户状态: True=正常, False=封禁"""
+
     storage: int = Field(default=0, sa_column_kwargs={"server_default": "0"})
     """已用存储空间（字节）"""
+
     two_factor: str | None = Field(default=None, max_length=255)
     """两步验证密钥"""
+
     avatar: str | None = Field(default=None, max_length=255)
     """头像地址"""
+
     options: str | None = Field(default=None)
     """用户个人设置 (JSON格式)"""
+
     authn: str | None = Field(default=None)
     """WebAuthn 凭证"""
+
     open_id: str | None = Field(default=None, max_length=255, unique=True, index=True)
     """第三方登录OpenID"""
+
     score: int = Field(default=0, sa_column_kwargs={"server_default": "0"})
     """用户积分"""
+
     group_expires: datetime | None = Field(default=None)
     """当前用户组过期时间"""
+
     phone: str | None = Field(default=None, max_length=255, unique=True, index=True)
     """手机号"""
 
     # 外键
-    group_id: int = Field(foreign_key="groups.id", index=True)
+    group_id: int = Field(foreign_key="group.id", index=True)
     """所属用户组ID"""
-    previous_group_id: int | None = Field(default=None, foreign_key="groups.id")
+
+    previous_group_id: int | None = Field(default=None, foreign_key="group.id")
     """之前的用户组ID（用于过期后恢复）"""
     
     # 关系
     group: "Group" = Relationship(
-        back_populates="users",
+        back_populates="user",
         sa_relationship_kwargs={
             "foreign_keys": "User.group_id"
         }
     )
     previous_group: Optional["Group"] = Relationship(
-        back_populates="previous_users",
+        back_populates="previous_user",
         sa_relationship_kwargs={
             "foreign_keys": "User.previous_group_id"
         }