OCR调试截图自动清理：从60张降至6张

- 微信和QQ截图统一池，合计最多6张
- 超过时从最早开始删除覆盖
- 确保保存失败时日志未打印也能通过截图及时发现问题

monitor.py

@@ -1249,7 +1249,7 @@ class MessageMonitor:
                 _dbg_path = os.path.join(script_dir, f"ocr_debug_qq_{ts}.png")
                 im.save(_dbg_path)
                 logger.debug(f"[qq] 调试截图: {_dbg_path}")
-                _clean_old_ocr_debug(script_dir, keep=60)
+                _clean_old_ocr_debug(script_dir, keep=6)
             except Exception as e:
                 logger.warning(f"[qq] 调试截图保存失败: {e} (路径: {os.path.join(script_dir, 'ocr_debug_qq_*.png')})")
 
@@ -1352,7 +1352,7 @@ class MessageMonitor:
                 im.save(_dbg_path)
                 logger.debug(f"[wechat] 调试截图: {_dbg_path}")
                 # 清理 ocr_debug_* 调试图（unified pool）
-                _clean_old_ocr_debug(script_dir, keep=60)
+                _clean_old_ocr_debug(script_dir, keep=6)
             except Exception as e:
                 logger.warning(f"[wechat] 调试截图保存失败: {e} (路径: {os.path.join(script_dir, 'ocr_debug_wx_*.png')})")