feat(events): add motion start/end flow with realtime client notifications

Backend/realtime/gateway.ts

@@ -31,6 +31,21 @@ const countSocketsForDevice = (deviceId: string): number => {
   return io.sockets.adapter.rooms.get(roomForDevice(deviceId))?.size ?? 0;
 };
 
+export const isDeviceOnline = (deviceId: string): boolean => countSocketsForDevice(deviceId) > 0;
+
+export const sendRealtimeToDevice = (
+  deviceId: string,
+  eventName: string,
+  payload: Record<string, unknown>,
+): boolean => {
+  if (!io || !isDeviceOnline(deviceId)) {
+    return false;
+  }
+
+  io.to(roomForDevice(deviceId)).emit(eventName, payload);
+  return true;
+};
+
 const markDevicePresence = async (deviceId: string, status: 'online' | 'offline') => {
   const now = new Date();