fix(iot-gateway): 修正RPC环境变量名为VIEWSH_IOT_GATEWAY_RPC_URL

Spring Boot relaxed binding将viewsh.iot.gateway.rpc.url映射为
VIEWSH_IOT_GATEWAY_RPC_URL，而非VIEWSH_GATEWAY_RPC_URL。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

viewsh-module-iot/viewsh-module-iot-gateway/src/main/resources/application-prod.yaml

@@ -36,7 +36,7 @@ viewsh:
     gateway:
       # 设备 RPC 配置
       rpc:
-        url: ${VIEWSH_GATEWAY_RPC_URL:http://aiot-iot-server:48091}
+        url: ${VIEWSH_IOT_GATEWAY_RPC_URL:http://aiot-iot-server:48091}
         connect-timeout: ${VIEWSH_RPC_CONNECT_TIMEOUT:30s}
         read-timeout: ${VIEWSH_RPC_READ_TIMEOUT:30s}
       # 设备 Token 配置