feat: 데모 사진 설정 프로퍼티 추가

Author: howooyeon

src/main/java/com/semosan/api/common/config/DemoProperties.java

@@ -0,0 +1,10 @@
+package com.semosan.api.common.config;
+
+import org.springframework.boot.context.properties.ConfigurationProperties;
+
+import java.util.List;
+
+@ConfigurationProperties(prefix = "demo")
+public record DemoProperties(
+        List<String> photoFilenames
+) {}