feat: 合并 File 和 Folder 模型为统一的 Object 模型，优化对象管理逻辑

refactor: 更新相关模型和路由以支持新对象模型，移除冗余代码

models/object.py

@@ -0,0 +1,185 @@
+
+from typing import TYPE_CHECKING, Optional
+from enum import StrEnum
+from sqlmodel import Field, Relationship, UniqueConstraint, CheckConstraint, Index
+from .base import TableBase
+
+if TYPE_CHECKING:
+    from .user import User
+    from .policy import Policy
+    from .source_link import SourceLink
+    from .share import Share
+
+
+class ObjectType(StrEnum):
+    """对象类型枚举"""
+    FILE = "file"
+    FOLDER = "folder"
+
+
+class Object(TableBase, table=True):
+    """
+    统一对象模型
+
+    合并了原有的 File 和 Folder 模型，通过 type 字段区分文件和目录。
+
+    根目录规则：
+    - 每个用户有一个显式根目录对象（name="~", parent_id=NULL）
+    - 用户创建的文件/文件夹的 parent_id 指向根目录或其他文件夹的 id
+    - 根目录的 policy_id 指定用户默认存储策略
+    """
+
+    __table_args__ = (
+        # 同一父目录下名称唯一（包括 parent_id 为 NULL 的情况）
+        UniqueConstraint("owner_id", "parent_id", "name", name="uq_object_parent_name"),
+        # 名称不能包含斜杠
+        CheckConstraint(
+            "name NOT LIKE '%/%' AND name NOT LIKE '%\\%'",
+            name="ck_object_name_no_slash",
+        ),
+        # 性能索引
+        Index("ix_object_owner_updated", "owner_id", "updated_at"),
+        Index("ix_object_parent_updated", "parent_id", "updated_at"),
+        Index("ix_object_owner_type", "owner_id", "type"),
+        Index("ix_object_owner_size", "owner_id", "size"),
+    )
+
+    # ==================== 基础字段 ====================
+
+    name: str = Field(max_length=255)
+    """对象名称（文件名或目录名）"""
+
+    type: ObjectType
+    """对象类型：file 或 folder"""
+
+    # ==================== 文件专属字段 ====================
+
+    source_name: str | None = None
+    """源文件名（仅文件有效）"""
+
+    size: int = Field(default=0, sa_column_kwargs={"server_default": "0"})
+    """文件大小（字节），目录为 0"""
+
+    upload_session_id: str | None = Field(default=None, max_length=255, unique=True, index=True)
+    """分块上传会话ID（仅文件有效）"""
+
+    file_metadata: str | None = None
+    """文件元数据 (JSON格式)，仅文件有效"""
+
+    # ==================== 外键 ====================
+
+    parent_id: int | None = Field(default=None, foreign_key="object.id", index=True)
+    """父目录ID，NULL 表示这是用户的根目录"""
+
+    owner_id: int = Field(foreign_key="user.id", index=True)
+    """所有者用户ID"""
+
+    policy_id: int = Field(foreign_key="policy.id", index=True)
+    """存储策略ID（文件直接使用，目录作为子文件的默认策略）"""
+
+    # ==================== 关系 ====================
+
+    owner: "User" = Relationship(back_populates="objects")
+    """所有者"""
+
+    policy: "Policy" = Relationship(back_populates="objects")
+    """存储策略"""
+
+    # 自引用关系
+    parent: Optional["Object"] = Relationship(
+        back_populates="children",
+        sa_relationship_kwargs={"remote_side": "Object.id"},
+    )
+    """父目录"""
+
+    children: list["Object"] = Relationship(back_populates="parent")
+    """子对象（文件和子目录）"""
+
+    # 仅文件有效的关系
+    source_links: list["SourceLink"] = Relationship(back_populates="object")
+    """源链接列表（仅文件有效）"""
+
+    shares: list["Share"] = Relationship(back_populates="object")
+    """分享列表"""
+
+    # ==================== 业务属性 ====================
+
+    @property
+    def is_file(self) -> bool:
+        """是否为文件"""
+        return self.type == ObjectType.FILE
+
+    @property
+    def is_folder(self) -> bool:
+        """是否为目录"""
+        return self.type == ObjectType.FOLDER
+
+    # ==================== 业务方法 ====================
+
+    @classmethod
+    async def get_root(cls, session, user_id: int) -> "Object | None":
+        """
+        获取用户的根目录
+
+        :param session: 数据库会话
+        :param user_id: 用户ID
+        :return: 根目录对象，不存在则返回 None
+        """
+        return await cls.get(
+            session,
+            (cls.owner_id == user_id) & (cls.parent_id == None)
+        )
+
+    @classmethod
+    async def get_by_path(cls, session, user_id: int, path: str) -> "Object | None":
+        """
+        根据路径获取对象
+
+        :param session: 数据库会话
+        :param user_id: 用户ID
+        :param path: 路径，如 "/" 或 "/docs/images"
+        :return: Object 或 None
+        """
+        path = path.strip()
+        if not path or path == "/" or path == "~":
+            return await cls.get_root(session, user_id)
+
+        # 移除开头的斜杠并分割路径
+        if path.startswith("/"):
+            path = path[1:]
+        parts = [p for p in path.split("/") if p]
+
+        if not parts:
+            return await cls.get_root(session, user_id)
+
+        # 从根目录开始遍历
+        current = await cls.get_root(session, user_id)
+
+        for part in parts:
+            if not current:
+                return None
+
+            current = await cls.get(
+                session,
+                (cls.owner_id == user_id) &
+                (cls.parent_id == current.id) &
+                (cls.name == part)
+            )
+
+        return current
+
+    @classmethod
+    async def get_children(cls, session, user_id: int, parent_id: int) -> list["Object"]:
+        """
+        获取目录下的所有子对象
+
+        :param session: 数据库会话
+        :param user_id: 用户ID
+        :param parent_id: 父目录ID
+        :return: 子对象列表
+        """
+        return await cls.get(
+            session,
+            (cls.owner_id == user_id) & (cls.parent_id == parent_id),
+            fetch_mode="all"
+        )