上传前人工裁剪错题区域，OCR 原文排除手写作答。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/annotation.py

@@ -107,3 +107,51 @@ def split_solution_sections(text: str) -> tuple[str | None, str]:
     approach = parts[0].replace("## 解题思路", "").strip()
     rest = "## " + parts[1]
     return approach or None, rest.strip()
+
+
+def union_bbox(bboxes: list[list[float]], img_w: int, img_h: int, padding_ratio: float = 0.06) -> list[int]:
+    if not bboxes:
+        return [0, 0, img_w, img_h]
+    x1 = min(b[0] for b in bboxes)
+    y1 = min(b[1] for b in bboxes)
+    x2 = max(b[2] for b in bboxes)
+    y2 = max(b[3] for b in bboxes)
+    pad_x = max(8, (x2 - x1) * padding_ratio)
+    pad_y = max(8, (y2 - y1) * padding_ratio)
+    return [
+        int(max(0, x1 - pad_x)),
+        int(max(0, y1 - pad_y)),
+        int(min(img_w, x2 + pad_x)),
+        int(min(img_h, y2 + pad_y)),
+    ]
+
+
+def cropped_rel_path(original_rel: str) -> str:
+    p = Path(original_rel)
+    return str(p.parent / f"{p.stem}_crop.jpg")
+
+
+def crop_wrong_region(
+    src_path: str,
+    lines: list[dict],
+    wrong_ids: list[int],
+    dest_rel_path: str,
+    img_width: int,
+    img_height: int,
+) -> str | None:
+    if not wrong_ids:
+        return None
+    bboxes = [lines[i].get("bbox") or [0, 0, 0, 0] for i in wrong_ids if i < len(lines)]
+    if not bboxes:
+        return None
+    box = union_bbox(bboxes, img_width, img_height, padding_ratio=0.12)
+    x1, y1, x2, y2 = box
+    if x2 <= x1 or y2 <= y1:
+        return None
+
+    img = Image.open(src_path).convert("RGB")
+    cropped = img.crop((x1, y1, x2, y2))
+    full_path = Path(settings.UPLOAD_DIR) / dest_rel_path
+    full_path.parent.mkdir(parents=True, exist_ok=True)
+    cropped.save(full_path, format="JPEG", quality=92)
+    return dest_rel_path