feat(client): 家属端设备列表返回在线状态与最后心跳时间

- 批量查询 Redis 心跳状态，避免逐台查询
- 修正心跳时间 Redis 序列化格式，防止读取时解析失败导致状态恒为离线

src/test/java/com/youlai/boot/device/DeviceOnlineRedisRoundTripTest.java

@@ -0,0 +1,45 @@
+package com.youlai.boot.device;
+
+import com.youlai.boot.config.JacksonConfig;
+import com.youlai.boot.device.model.vo.DeviceOnlineVO;
+import org.junit.jupiter.api.Test;
+import org.springframework.data.redis.serializer.JacksonJsonRedisSerializer;
+import tools.jackson.databind.cfg.DateTimeFeature;
+import tools.jackson.databind.json.JsonMapper;
+
+import java.time.LocalDateTime;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * 设备在线状态在 Redis 写入与全局 ObjectMapper 读取之间的格式一致性校验
+ * <p>
+ * RedisConfig 的序列化器与 JacksonConfig 的全局时间格式不一致时，
+ * convertValue 会抛 DateTimeParseException，导致在线状态永远读成离线。
+ */
+class DeviceOnlineRedisRoundTripTest {
+
+    @Test
+    void redisValueCanBeConvertedByGlobalObjectMapper() {
+        JsonMapper redisMapper = JsonMapper.builder()
+                .disable(DateTimeFeature.WRITE_DATES_AS_TIMESTAMPS)
+                .build();
+        JacksonJsonRedisSerializer<Object> serializer = new JacksonJsonRedisSerializer<>(redisMapper, Object.class);
+
+        LocalDateTime heartbeat = LocalDateTime.of(2026, 11, 7, 10, 37, 22);
+
+        DeviceOnlineVO vo = new DeviceOnlineVO();
+        vo.setSerialno("895Y044WC");
+        vo.setOnline(1);
+        vo.setIp("192.168.100.141");
+        vo.setScreenState(0);
+        vo.setLastHeartbeatTime(heartbeat);
+
+        Object cached = serializer.deserialize(serializer.serialize(vo));
+        DeviceOnlineVO converted = new JacksonConfig().objectMapper().convertValue(cached, DeviceOnlineVO.class);
+
+        assertEquals("895Y044WC", converted.getSerialno());
+        assertEquals(1, converted.getOnline());
+        assertEquals(heartbeat, converted.getLastHeartbeatTime());
+    }
+}