feat(backend): report node platform, geo and build info

Agents and probes report OS, arch and build (version, tag, commit, time).
The backend looks up country and ISP of public addresses only, stores them
in node_meta (schema v5) and exposes display info via /api/public/nodes,
/api/public/about and /api/admin/build-info behind site.show_build.

backend/tests/Monitor.Backend.Tests/BackendFixture.cs

@@ -192,6 +192,7 @@ public class BackendFixture : IAsyncLifetime
                 options.InitialAdminUser = AdminUser;
                 options.InitialAdminPassword = AdminPassword;
                 options.LoginAttemptsPerMinute = LoginAttemptsPerMinute;
+                options.GeoLookupUrl = string.Empty; // 测试里不访问外部服务
                 options.PhaseDurationScale = 0.2; // 端到端测试里 connect、hold 这些固定 30 秒的阶段缩成 6 秒
             });
         await App.StartAsync();