fix: TaskRecordResponse改用field_validator — 不再污染ORM对象导致图片列表写回VARCHAR报错

根因: model_validate直接obj.images=json.loads()把ORM对象的字符串改成列表
     SQLAlchemy autoflush时把列表写回VARCHAR列 → DataError
修复: field_validator(mode=before)在序列化层转换, 不修改ORM源对象

backend/app/schemas/task.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import uuid
 from datetime import datetime
 from typing import Optional
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 
 
 # ============================================================
@@ -82,18 +82,19 @@ class TaskRecordResponse(BaseModel):
 
     model_config = {"from_attributes": True}
 
+    @field_validator("images", mode="before")
     @classmethod
-    def model_validate(cls, obj, **kwargs):
-        """处理 DB 中 images 的 JSON 字符串 → list 反序列化"""
+    def _parse_images(cls, v):
+        """处理 DB 中 images 的 JSON 字符串 → list 反序列化（不污染 ORM 对象）"""
         import json
-        if hasattr(obj, "images") and isinstance(obj.images, str):
+        if isinstance(v, str):
             try:
-                obj.images = json.loads(obj.images)
+                return json.loads(v)
             except (json.JSONDecodeError, TypeError):
-                obj.images = []
-        elif hasattr(obj, "images") and obj.images is None:
-            obj.images = []
-        return super().model_validate(obj, **kwargs)
+                return []
+        if v is None:
+            return []
+        return v
 
 
 # ============================================================