feat(docs): 添加设备心跳表SQL文档

新增TDengine超级表device_heartbeat的建表语句及EMQX规则引擎配置示例。

docs/sql/device_status_log.sql

@@ -0,0 +1,43 @@
+USE iot_log;
+
+-- 创建同时记录上下线状态的超级表
+CREATE STABLE IF NOT EXISTS device_status_log (
+    ts TIMESTAMP,         -- 事件发生时间
+    event VARCHAR(16),    -- 事件类型: 'connected' 或 'disconnected'
+    reason VARCHAR(64),   -- 下线原因（上线时填 'clean_start' 或 'none'）
+    peername VARCHAR(64)  -- 客户端 IP 及端口
+) TAGS (
+    clientid VARCHAR(128),
+    username VARCHAR(128)
+);
+
+-- emqx 设备离线 rules
+SELECT
+    clientid,
+    username,
+    peername,
+    reason,
+    disconnected_at AS ts,
+    'disconnected' AS event
+FROM
+    "$events/client_disconnected"
+
+-- emqx 设备离线 action
+INSERT INTO mqtt.device_${clientid}
+USING mqtt.device_status_log TAGS ('${clientid}', '${username}')
+VALUES (${ts}, '${event}', '${reason}', '${peername}');
+
+-- emqx 设备上线 rules
+SELECT
+    clientid,
+    username,
+    peername,
+    connected_at AS ts,
+    'connected' AS event
+FROM
+    "$events/client_connected"
+
+-- emqx 设备上线 action
+INSERT INTO mqtt.device_${clientid}
+USING mqtt.device_status_log TAGS ('${clientid}', '${username}')
+VALUES (${ts}, '${event}', 'none', '${peername}');