作业帮式错题标注：OCR 定位错误红框 + 解题思路。

- PaddleOCR 行级坐标 + AI 识别错答区域，生成标注图

- 解法拆分为「解题思路」与「详细解答」

- 详情页标注图/原图切换，列表显示标注缩略图

backend/app/schemas/__init__.py

@@ -233,7 +233,10 @@ class WrongQuestionOut(BaseModel):
     image_path: str
     ocr_raw_text: str | None
     question_text: str | None
+    solution_approach: str | None = None
     solution_text: str | None
+    mark_regions: list[dict] | None = None
+    has_annotated_image: bool = False
     status: WrongQuestionStatusEnum
     created_at: datetime
 
@@ -242,5 +245,6 @@ class WrongQuestionOut(BaseModel):
 
 class WrongQuestionUpdate(BaseModel):
     question_text: str | None = None
+    solution_approach: str | None = None
     solution_text: str | None = None
     subject_id: int | None = None