fix: dispatcher 串行上传 + 看门狗自动重启 + 线程状态真实检测

- Dispatcher limit=10→1: 一次只传一个视频，传完再传下一个
- 退避缩短: min(30*(n+1),300)s → 失败后更快重试（原 min(60*(n+1)*2,600)s）
- /api/status: 用 thread.is_alive() 替代 _running 布尔标志
- 三组件(scheduler/dispatcher/poller)添加 is_alive()+check_and_restart()
- app.py 新增 watchdog 线程: 每 60s 检测线程死亡并自动重启

fam-core/src/fam_core/scheduler/scheduler.py

@@ -104,6 +104,17 @@ class TaskScheduler:
         self._thread = threading.Thread(target=self._run, daemon=True, name='task-scheduler')
         self._thread.start()
 
+    def is_alive(self):
+        """线程是否存活"""
+        return self._thread is not None and self._thread.is_alive()
+
+    def check_and_restart(self):
+        """看门狗：线程崩溃后自动重启"""
+        if self._running and not self.is_alive():
+            logger.warning("Scheduler 线程已死亡，正在重启...")
+            self._thread = threading.Thread(target=self._run, daemon=True, name='task-scheduler')
+            self._thread.start()
+
     def stop(self):
         """停止调度线程"""
         self._running = False