refactor(沙箱工具): 将沙箱镜像配置改为从settings读取

默认镜像配置改为从settings.SANDBOX_IMAGE读取，提高配置灵活性

backend/app/services/agent/tools/sandbox_tool.py

@@ -14,6 +14,7 @@ from pydantic import BaseModel, Field
 from dataclasses import dataclass
 
 from .base import AgentTool, ToolResult
+from app.core.config import settings
 
 logger = logging.getLogger(__name__)
 
@@ -21,7 +22,7 @@ logger = logging.getLogger(__name__)
 @dataclass
 class SandboxConfig:
     """沙箱配置"""
-    image: str = "deepaudit/sandbox:latest"
+    image: str = None  # 默认从 settings.SANDBOX_IMAGE 读取
     memory_limit: str = "512m"
     cpu_limit: float = 1.0
     timeout: int = 60
@@ -29,6 +30,10 @@ class SandboxConfig:
     read_only: bool = True
     user: str = "1000:1000"
 
+    def __post_init__(self):
+        if self.image is None:
+            self.image = settings.SANDBOX_IMAGE
+
 
 class SandboxManager:
     """