添加渲染失败日志记录便于回溯

# 升级渲染失败日志：记录 LaTeX 内容

## 问题
当前日志只记录 `ID`，无法追溯失败的 LaTeX 公式。

## 修复（3 行）
```python
# generate_formula_images.py
except Exception as e:
    latex_preview = latex.replace('\n', ' ')[:50]
    logging.warning(f"{id} | LaTeX: {latex_preview}... | Error: {e}")
```
*This issue was proposed with assistance from **Grok** (by xAI).*