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/ocr.py

@@ -1,5 +1,7 @@
 from pathlib import Path
 
+from PIL import Image
+
 from app.core.config import settings
 
 _ocr_engine = None
@@ -14,18 +16,52 @@ def get_ocr_engine():
     return _ocr_engine
 
 
-def run_ocr(image_path: str) -> str:
+def _bbox_from_box(box: list) -> list[float]:
+    xs = [float(p[0]) for p in box]
+    ys = [float(p[1]) for p in box]
+    return [min(xs), min(ys), max(xs), max(ys)]
+
+
+def run_ocr_with_regions(image_path: str) -> dict:
+    """Return OCR text plus line-level bounding boxes for annotation."""
     engine = get_ocr_engine()
     result = engine.ocr(image_path, cls=True)
-    if not result or not result[0]:
-        return ""
-    lines = []
-    for line in result[0]:
-        if line and len(line) >= 2:
-            text = line[1][0]
-            if text:
-                lines.append(text)
-    return "\n".join(lines)
+    lines: list[dict] = []
+    if result and result[0]:
+        for item in result[0]:
+            if not item or len(item) < 2:
+                continue
+            box, rec = item[0], item[1]
+            text = rec[0] if rec else ""
+            conf = float(rec[1]) if rec and len(rec) > 1 else 0.0
+            if not text:
+                continue
+            lines.append(
+                {
+                    "text": text,
+                    "confidence": conf,
+                    "box": box,
+                    "bbox": _bbox_from_box(box),
+                }
+            )
+
+    width, height = 0, 0
+    try:
+        with Image.open(image_path) as img:
+            width, height = img.size
+    except OSError:
+        pass
+
+    return {
+        "text": "\n".join(line["text"] for line in lines),
+        "lines": lines,
+        "width": width,
+        "height": height,
+    }
+
+
+def run_ocr(image_path: str) -> str:
+    return run_ocr_with_regions(image_path)["text"]
 
 
 def save_upload_file(user_id: str, question_id: str, filename: str, content: bytes) -> str:
@@ -38,3 +74,8 @@ def save_upload_file(user_id: str, question_id: str, filename: str, content: byt
     full_path = Path(settings.UPLOAD_DIR) / rel_path
     full_path.write_bytes(content)
     return rel_path
+
+
+def annotated_rel_path(original_rel: str) -> str:
+    p = Path(original_rel)
+    return str(p.parent / f"{p.stem}_marked.jpg")