修复环境变量配置：使用APP_DOMAIN代替app.domain以兼容systemd

Author: Layau-code

src/main/java/com/layor/tinyflow/service/ShortUrlService.java

@@ -64,7 +64,7 @@ public class ShortUrlService {
     @Autowired
     private AuthService authService; // 注入认证服务
 
-    @Value("${app.domain:http://localhost:8080}")
+    @Value("${APP_DOMAIN:http://localhost:8080}")
     private String baseUrl;
 
     public ShortUrlDTO createShortUrl(String longUrl, String customAlias) throws Exception {