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/services/migrate.py

@@ -65,3 +65,17 @@ def run_migrations() -> None:
             with engine.begin() as conn:
                 for clause in alters:
                     conn.execute(text(f"ALTER TABLE system_settings {clause}"))
+
+    if "wrong_questions" in tables:
+        wq_columns = {col["name"] for col in inspector.get_columns("wrong_questions")}
+        wq_alters: list[str] = []
+        if "solution_approach" not in wq_columns:
+            wq_alters.append("ADD COLUMN solution_approach TEXT")
+        if "mark_regions_json" not in wq_columns:
+            wq_alters.append("ADD COLUMN mark_regions_json TEXT")
+        if "annotated_image_path" not in wq_columns:
+            wq_alters.append("ADD COLUMN annotated_image_path VARCHAR(512)")
+        if wq_alters:
+            with engine.begin() as conn:
+                for clause in wq_alters:
+                    conn.execute(text(f"ALTER TABLE wrong_questions {clause}"))