feat(sites): auto-screenshot on final download failure for debugging

所有站点 with_retry 在最后一次重试失败、reset 之前自动截图，
保存到 logs/screenshots/。网页站点走 Playwright page.screenshot()，
安能走 CDP Page.captureScreenshot。截图失败绝不阻塞任务流程。

- paths.py: 新增 SCREENSHOT_DIR (BASE_DIR/logs/screenshots/)
- runtime.py: 新增 capture_error_screenshot() 工具函数
- .gitignore: 新增 logs/ 忽略规则

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -42,6 +42,7 @@ desktop.ini
 downloads/
 output/
 state/
+logs/
 *.xlsx
 *.xls
 *.log