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/src/Monitor.Backend.Api/Contracts/ApiJsonContext.cs

@@ -37,6 +37,12 @@ namespace Monitor.Backend.Api;
 [JsonSerializable(typeof(AgentDto))]
 [JsonSerializable(typeof(PageDto<AgentDto>))]
 [JsonSerializable(typeof(AgentPatchRequest))]
+[JsonSerializable(typeof(BuildDto))]
+[JsonSerializable(typeof(AboutDto))]
+[JsonSerializable(typeof(NodeBuildDto))]
+[JsonSerializable(typeof(BuildInfoDto))]
+[JsonSerializable(typeof(NodeInfoDto))]
+[JsonSerializable(typeof(PageDto<NodeInfoDto>))]
 [JsonSerializable(typeof(JoinTokenDto))]
 [JsonSerializable(typeof(PageDto<JoinTokenDto>))]
 [JsonSerializable(typeof(CreateJoinTokenRequest))]