feat(edge): 截图支持临时 RTSP 连接，解决无 ROI 摄像头无法截图

_capture_frame 增加 rtsp_url 参数，优先走已有流，无流时降级
临时连接 RTSP 抓帧（_capture_ondemand），用完即释放。
提取 _encode_jpeg 公共方法。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/screenshot_handler.py

@@ -181,11 +181,13 @@ class ScreenshotHandler:
             })
             return
 
+        rtsp_url = fields.get("rtsp_url", "")
+
         logger.info("[截图] 收到截图请求: request_id=%s, camera=%s, callback=%s",
                     request_id, camera_code, callback_url or "(无)")
 
         # 1. 抓帧
-        jpeg_bytes = self._capture_frame(camera_code)
+        jpeg_bytes = self._capture_frame(camera_code, rtsp_url)
         if jpeg_bytes is None:
             self._send_result(callback_url, request_id, camera_code, {
                 "status": "error",
@@ -217,36 +219,62 @@ class ScreenshotHandler:
 
     # ==================== 抓帧 ====================
 
-    def _capture_frame(self, camera_code: str) -> Optional[bytes]:
-        """从 MultiStreamManager 获取最新帧并编码为 JPEG"""
+    def _capture_frame(self, camera_code: str, rtsp_url: str = "") -> Optional[bytes]:
+        """从 MultiStreamManager 获取最新帧，无流时降级临时 RTSP 连接"""
+        # 优先从已有视频流获取（有 ROI 的摄像头，已连接）
         stream = self._stream_manager.get_stream(camera_code)
-        if stream is None:
-            logger.warning("[截图] 未找到摄像头流: %s", camera_code)
-            return None
+        if stream is not None and stream.is_connected:
+            frame = stream.get_latest_frame(timeout=2.0)
+            if frame is None:
+                frame = stream.read(timeout=2.0)
+            if frame is not None:
+                return self._encode_jpeg(frame.image)
 
-        if not stream.is_connected:
-            logger.warning("[截图] 摄像头未连接: %s", camera_code)
-            return None
+        # 降级：临时 RTSP 连接抓帧（无 ROI 或流未连接的摄像头）
+        if rtsp_url:
+            return self._capture_ondemand(camera_code, rtsp_url)
 
-        frame = stream.get_latest_frame(timeout=2.0)
-        if frame is None:
-            # 回退：直接从缓冲区读一帧
-            frame = stream.read(timeout=2.0)
+        logger.warning("[截图] 未找到摄像头流且无 rtsp_url: %s", camera_code)
+        return None
 
-        if frame is None:
-            logger.warning("[截图] 获取帧超时: %s", camera_code)
-            return None
-
-        # JPEG 编码
+    def _capture_ondemand(self, camera_code: str, rtsp_url: str) -> Optional[bytes]:
+        """临时连接 RTSP 抓取一帧，用完即断"""
+        cap = None
         try:
-            encode_params = [cv2.IMWRITE_JPEG_QUALITY, 85]
-            success, buffer = cv2.imencode(".jpg", frame.image, encode_params)
-            if not success:
-                logger.error("[截图] JPEG 编码失败: %s", camera_code)
+            logger.info("[截图] 临时连接 RTSP: %s → %s", camera_code, rtsp_url)
+            cap = cv2.VideoCapture(rtsp_url, cv2.CAP_FFMPEG)
+            cap.set(cv2.CAP_PROP_BUFFERSIZE, 1)
+            if not cap.isOpened():
+                logger.warning("[截图] 临时 RTSP 连接失败: %s", camera_code)
                 return None
-            return buffer.tobytes()
+
+            # 读取几帧丢弃（跳过关键帧解码延迟），取最后一帧
+            frame = None
+            for _ in range(5):
+                ret, f = cap.read()
+                if ret and f is not None:
+                    frame = f
+
+            if frame is None:
+                logger.warning("[截图] 临时连接读帧失败: %s", camera_code)
+                return None
+
+            return self._encode_jpeg(frame)
         except Exception as e:
-            logger.error("[截图] 帧编码异常: %s", e)
+            logger.error("[截图] 临时截图异常: %s, %s", camera_code, e)
+            return None
+        finally:
+            if cap is not None:
+                cap.release()
+                logger.debug("[截图] 临时 RTSP 连接已释放: %s", camera_code)
+
+    def _encode_jpeg(self, image) -> Optional[bytes]:
+        """将帧编码为 JPEG 字节"""
+        try:
+            success, buffer = cv2.imencode(".jpg", image, [cv2.IMWRITE_JPEG_QUALITY, 85])
+            return buffer.tobytes() if success else None
+        except Exception as e:
+            logger.error("[截图] JPEG 编码失败: %s", e)
             return None
 
     # ==================== COS 上传 ====================