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/Admin/BuildEndpoints.cs

@@ -0,0 +1,30 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Routing;
+using Monitor.Backend.Application;
+
+namespace Monitor.Backend.Api;
+
+/// <summary>管理端的构建信息：后端自己和每个 Agent、Probe 的版本、tag、提交、构建时间（节点上报的，不含任何地址）。</summary>
+public static class BuildEndpoints
+{
+    private const string Tag = "admin-build";
+
+    public static void Map(RouteGroupBuilder group)
+    {
+        group.MapGet("/build-info", GetAsync)
+            .Api("build_info_get", "Version, tag, commit and build time of the backend and every agent and probe", Tag)
+            .Returns<BuildInfoDto>().ProducesProblems(401, 403);
+    }
+
+    private static async Task<IResult> GetAsync(HttpContext http, NodeInfoService nodes, CancellationToken ct)
+    {
+        var items = (await nodes.ListAsync(ct))
+            .OrderBy(n => n.Kind, StringComparer.Ordinal)
+            .ThenBy(n => n.NodeId, StringComparer.Ordinal)
+            .Select(n => new NodeBuildDto(n.NodeId, n.Kind, n.Os, n.OsName, n.Arch, n.Version, n.Tag, n.Commit, n.BuiltAt))
+            .ToList();
+        http.Response.Headers.CacheControl = "no-store";
+        return TypedResults.Json(new BuildInfoDto(NodeEndpoints.Backend(withCommit: true), items), ApiJsonContext.Default.BuildInfoDto);
+    }
+}