feat: 优化设备冷却管理，增加原子设备占用逻辑和过期记录处理

src/main/resources/mapper/agent/LinkTaskMapper.xml

@@ -255,4 +255,25 @@
         WHERE status = #{status}
         ORDER BY created_at ASC
     </select>
+
+    <!-- 原子占用设备，避免并发下同一设备被多个链接占用 -->
+    <update id="reserveDeviceIfFree">
+        UPDATE link_task lt
+        LEFT JOIN (
+            SELECT 1 as has_conflict
+            FROM link_task x
+            WHERE x.machine_id = #{deviceId}
+              AND x.status IN ('USING','LOGGED_IN')
+              AND x.id &lt;&gt; #{id}
+        ) conflict_check ON 1=1
+        SET lt.status = 'USING',
+            lt.region = #{region},
+            lt.machine_id = #{deviceId},
+            lt.qr_created_at = NOW(),
+            lt.qr_expire_at = DATE_ADD(NOW(), INTERVAL #{qrExpireSeconds} SECOND),
+            lt.updated_at = NOW()
+        WHERE lt.id = #{id}
+          AND conflict_check.has_conflict IS NULL
+    </update>
+
 </mapper>