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/models/user.py

@@ -129,7 +129,10 @@ class WrongQuestion(Base):
     image_path: Mapped[str] = mapped_column(String(512))
     ocr_raw_text: Mapped[str | None] = mapped_column(Text, nullable=True)
     question_text: Mapped[str | None] = mapped_column(Text, nullable=True)
+    solution_approach: Mapped[str | None] = mapped_column(Text, nullable=True)
     solution_text: Mapped[str | None] = mapped_column(Text, nullable=True)
+    mark_regions_json: Mapped[str | None] = mapped_column(Text, nullable=True)
+    annotated_image_path: Mapped[str | None] = mapped_column(String(512), nullable=True)
     status: Mapped[WrongQuestionStatus] = mapped_column(
         Enum(WrongQuestionStatus), default=WrongQuestionStatus.pending
     )