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/annotation.py

@@ -0,0 +1,109 @@
+import json
+import re
+from pathlib import Path
+
+from PIL import Image, ImageDraw, ImageFont
+
+from app.core.config import settings
+
+
+def _parse_llm_json(text: str) -> dict | None:
+    text = text.strip()
+    match = re.search(r"\{[\s\S]*\}", text)
+    if not match:
+        return None
+    try:
+        return json.loads(match.group())
+    except json.JSONDecodeError:
+        return None
+
+
+def heuristic_wrong_line_ids(lines: list[dict]) -> list[int]:
+    wrong: list[int] = []
+    for i, line in enumerate(lines):
+        t = line.get("text", "")
+        if any(c in t for c in ("×", "✗", "❌", "错")):
+            wrong.append(i)
+            continue
+        if re.search(r"[×xX]\s*$", t.strip()):
+            wrong.append(i)
+    if wrong:
+        return wrong
+    # 单题照片：标注最后几行作答区域
+    if len(lines) == 1:
+        return [0]
+    if len(lines) <= 4:
+        return list(range(max(0, len(lines) - 2), len(lines)))
+    return list(range(len(lines) - 2, len(lines)))
+
+
+def parse_wrong_line_ids(llm_response: str, lines: list[dict]) -> list[int]:
+    data = _parse_llm_json(llm_response)
+    if data and isinstance(data.get("wrong_line_ids"), list):
+        ids = [int(x) for x in data["wrong_line_ids"] if isinstance(x, (int, float, str))]
+        ids = [i for i in ids if 0 <= i < len(lines)]
+        if ids:
+            return ids
+    return heuristic_wrong_line_ids(lines)
+
+
+def regions_from_lines(lines: list[dict], wrong_ids: list[int]) -> list[dict]:
+    regions = []
+    for i in wrong_ids:
+        if i >= len(lines):
+            continue
+        line = lines[i]
+        bbox = line.get("bbox") or [0, 0, 0, 0]
+        regions.append(
+            {
+                "line_id": i,
+                "text": line.get("text", ""),
+                "bbox": bbox,
+                "type": "wrong",
+                "label": "错",
+            }
+        )
+    return regions
+
+
+def draw_annotated_image(
+    src_path: str,
+    lines: list[dict],
+    wrong_ids: list[int],
+    dest_rel_path: str,
+) -> str:
+    img = Image.open(src_path).convert("RGBA")
+    overlay = Image.new("RGBA", img.size, (255, 255, 255, 0))
+    draw = ImageDraw.Draw(overlay)
+
+    try:
+        font = ImageFont.truetype("DejaVuSans-Bold.ttf", max(14, img.size[0] // 40))
+    except OSError:
+        font = ImageFont.load_default()
+
+    for i in wrong_ids:
+        if i >= len(lines):
+            continue
+        bbox = lines[i].get("bbox") or [0, 0, 0, 0]
+        x1, y1, x2, y2 = bbox
+        pad = 6
+        box = [x1 - pad, y1 - pad, x2 + pad, y2 + pad]
+        draw.rounded_rectangle(box, radius=4, fill=(255, 59, 48, 55), outline=(255, 59, 48, 220), width=3)
+        draw.text((x1, max(0, y1 - 18)), "×", fill=(255, 59, 48, 255), font=font)
+
+    combined = Image.alpha_composite(img, overlay).convert("RGB")
+    full_path = Path(settings.UPLOAD_DIR) / dest_rel_path
+    full_path.parent.mkdir(parents=True, exist_ok=True)
+    combined.save(full_path, format="JPEG", quality=92)
+    return dest_rel_path
+
+
+def split_solution_sections(text: str) -> tuple[str | None, str]:
+    if "## 解题思路" not in text:
+        return None, text
+    parts = re.split(r"\n##\s*", text, maxsplit=1)
+    if len(parts) < 2:
+        return None, text
+    approach = parts[0].replace("## 解题思路", "").strip()
+    rest = "## " + parts[1]
+    return approach or None, rest.strip()