perf+fix(danding_qqpush): perf优化+安全修复+代码DRY

- image_render: cached draw object, font.getlength() 替代逐字符创建临时Image
- image_render: 移除PNG无效的quality参数
- api.py: ImageRenderer单例复用(避免每请求重载字体)
- api.py: 异常详情不再泄露到API响应
- sender.py: 提取_send_msg()消除重复代码

danding_bot/plugins/danding_qqpush/api.py

@@ -8,6 +8,9 @@ from nonebot import get_driver, logger
 from .config import Config
 from .text_parser import TextParser
 from .image_render import ImageRenderer
+
+# Module-level singleton: load font once, reuse across requests
+_renderer = _renderer  # reuse module-level singleton
 from .sender import sender
 
 
@@ -134,10 +137,10 @@ def create_routes(token: str, config: Config):
             raise
         
         except Exception as e:
-            logger.exception(f"推送接口异常: {str(e)}")
+            logger.exception(f"推送接口异常: {e}")
             raise HTTPException(
                 status_code=500,
-                detail=f"服务器内部错误: {str(e)}"
+                detail="服务器内部错误"
             )
     
     return router