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/source_link.py

@@ -1,24 +1,29 @@
 
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING
 from sqlmodel import Field, Relationship, Index
 from .base import TableBase
-from datetime import datetime
 
 if TYPE_CHECKING:
-    from .file import File
+    from .object import Object
+
 
 class SourceLink(TableBase, table=True):
     """链接模型"""
 
     __table_args__ = (
-        Index("ix_sourcelink_file_name", "file_id", "name"),
+        Index("ix_sourcelink_object_name", "object_id", "name"),
     )
 
-    name: str = Field(max_length=255, description="链接名称")
-    downloads: int = Field(default=0, sa_column_kwargs={"server_default": "0"}, description="通过此链接的下载次数")
-    
+    name: str = Field(max_length=255)
+    """链接名称"""
+
+    downloads: int = Field(default=0, sa_column_kwargs={"server_default": "0"})
+    """通过此链接的下载次数"""
+
     # 外键
-    file_id: int = Field(foreign_key="file.id", index=True, description="关联的文件ID")
-    
+    object_id: int = Field(foreign_key="object.id", index=True)
+    """关联的对象ID（必须是文件类型）"""
+
     # 关系
-    file: "File" = Relationship(back_populates="source_links")
+    object: "Object" = Relationship(back_populates="source_links")
+    """关联的对象"""