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/agent/src/Monitor.Agent/AgentApp.cs

@@ -12,7 +12,7 @@ namespace Monitor.Agent;
 /// <summary>Agent 的组装点：数据面、点对点监听、与 Backend 的链路都在这里接起来。没有全局状态，测试可以在同一进程里启动多个。</summary>
 public sealed class AgentApp : IDisposable
 {
-    public const string Version = "0.1.1";
+    public const string Version = Monitor.Protocol.BuildInfo.Version;
 
     private readonly AgentConfig _config;
     private readonly LoadedNodeIdentity _identity;
@@ -244,6 +244,7 @@ public sealed class AgentApp : IDisposable
     {
         var meta = new AgentHelloMeta
         {
+            Platform = Monitor.Measurement.PlatformDetector.Detect(),
             TlsSpki = _identity.File.TlsSpki,
             ProbeTcpPort = ControlPort,
             ProbeUdpPorts = [.. UdpPorts],

backend/agent/src/Monitor.Agent/DataPlane/Registries.cs

@@ -64,6 +64,9 @@ public sealed class ProbeSession : DataSession
 
     public long ConnectedAtUtcMs { get; init; }
 
+    /// <summary>Probe 自报的系统和架构（随列表同步一起来，只用于展示）；还没报过就是 null。</summary>
+    public PlatformInfo? Platform { get; set; }
+
     public RpcSession<PeerContext>? Rpc { get; set; }
 
     /// <summary>最近一次认证通过的 UDP 包的源地址（Agent 把单向数据发往这里）。</summary>

backend/agent/src/Monitor.Agent/Peer/PeerHub.cs

@@ -220,7 +220,7 @@ public sealed class PeerHub
     /// <summary>当前连着的 Probe（心跳和 Backend 重连之后的补报都用它）。</summary>
     public IReadOnlyList<ConnectedProbe> ConnectedProbes() => [.. _sessions.All.Select(Describe)];
 
-    private static ConnectedProbe Describe(ProbeSession p) => new() { Id = new ProbeId(p.ProbeId), Version = p.Version, Remote = p.RemoteAddress, SinceUtcMs = p.ConnectedAtUtcMs };
+    private static ConnectedProbe Describe(ProbeSession p) => new() { Id = new ProbeId(p.ProbeId), Version = p.Version, Remote = p.RemoteAddress, SinceUtcMs = p.ConnectedAtUtcMs, Platform = p.Platform };
 
     public IReadOnlyList<string> ConnectedProbeIds() => [.. _sessions.All.Select(p => p.ProbeId)];
 
@@ -306,7 +306,15 @@ public sealed class PeerHub
                 MonitorMethods.ProbeAgents,
                 MonitorJsonContext.Default.ProbeAgentsParams,
                 MonitorJsonContext.Default.ProbeAgentsResult,
-                (_, request, ct) => uplink.RequestProbeAgentsAsync(request, ct))
+                (context, request, ct) =>
+                {
+                    if (request.Platform is { } platform)
+                    {
+                        context.Session.Platform = platform;
+                    }
+
+                    return uplink.RequestProbeAgentsAsync(request, ct);
+                })
             .On(
                 MonitorMethods.Clock,
                 MonitorJsonContext.Default.ClockParams,