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/shared/Monitor.Protocol/Rpc/MonitorRpc.cs

@@ -152,6 +152,10 @@ public sealed class ProbeAgentsParams
     /// <summary>Probe 目前持有的列表版本（用于调试；服务端总是返回当前列表）。</summary>
     [JsonPropertyName("have_version")]
     public long HaveVersion { get; set; }
+
+    /// <summary>Probe 的系统和架构（只用于展示；Agent 记在会话上，随连接状态报给 Backend）。</summary>
+    [JsonPropertyName("platform")]
+    public PlatformInfo? Platform { get; set; }
 }
 
 public sealed class ProbeAgentsResult
@@ -194,9 +198,41 @@ public sealed class LocalLimits
     public int MaxConcurrentTests { get; set; } = 1;
 }
 
+/// <summary>
+/// 节点的系统和架构（粗粒度，只用于展示）：<c>os</c> 是 linux / windows / macos / other，<c>name</c> 是「Debian 12」「Windows 11」这样的名字，
+/// <c>arch</c> 是 x64 / arm64 / arm / x86。<b>没有主机名、内核版本、具体型号</b>。另外带着这个程序自己的构建信息（版本、tag、构建时间）。
+/// </summary>
+public sealed class PlatformInfo
+{
+    [JsonPropertyName("os")]
+    public string Os { get; set; } = "other";
+
+    [JsonPropertyName("name")]
+    public string? Name { get; set; }
+
+    [JsonPropertyName("arch")]
+    public string? Arch { get; set; }
+
+    /// <summary>程序的版本、tag、提交（短）和构建时间（UTC，ISO 8601）：<see cref="BuildInfo"/> 构建时写进去的，没设置就是空。</summary>
+    [JsonPropertyName("version")]
+    public string? Version { get; set; }
+
+    [JsonPropertyName("tag")]
+    public string? Tag { get; set; }
+
+    [JsonPropertyName("commit")]
+    public string? Commit { get; set; }
+
+    [JsonPropertyName("built_at")]
+    public string? BuiltAt { get; set; }
+}
+
 /// <summary><c>fleet.hello</c> 的附加字段。</summary>
 public sealed class AgentHelloMeta
 {
+    [JsonPropertyName("platform")]
+    public PlatformInfo? Platform { get; set; }
+
     [JsonPropertyName("tls_spki")]
     public string? TlsSpki { get; set; }
 
@@ -266,6 +302,9 @@ public sealed class ConnectedProbe
 
     [JsonPropertyName("since_utc_ms")]
     public long SinceUtcMs { get; set; }
+
+    [JsonPropertyName("platform")]
+    public PlatformInfo? Platform { get; set; }
 }
 
 /// <summary><c>fleet.heartbeat</c> 的附加字段：只有进程内的计数，没有主机信息。</summary>
@@ -434,6 +473,7 @@ public static class FragmentKeys
 [JsonSerializable(typeof(ProbeAgentsParams))]
 [JsonSerializable(typeof(ProbeAgentsResult))]
 [JsonSerializable(typeof(AgentEnrollMeta))]
+[JsonSerializable(typeof(PlatformInfo))]
 [JsonSerializable(typeof(AgentHelloMeta))]
 [JsonSerializable(typeof(AgentHeartbeatExtra))]
 [JsonSerializable(typeof(RawChunkHeader))]