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/Monitor.Protocol.csproj

@@ -6,4 +6,30 @@
     <ProjectReference Include="..\..\fleet\src\Fleet.Core\Fleet.Core.csproj" />
     <ProjectReference Include="..\..\fleet\src\Fleet.Rpc\Fleet.Rpc.csproj" />
   </ItemGroup>
+  <!--
+    构建信息：版本来自 $(Version)，tag、提交、构建时间来自环境变量 MONITOR_TAG / MONITOR_COMMIT / MONITOR_BUILT_AT
+    （scripts/publish.sh 和 CI 会设置；本地开发不设置就是空，这样本地构建是可重复的、不会每次都重新编译）。
+    生成一个带常量的源文件，NativeAOT 下不用反射。
+  -->
+  <Target Name="WriteBuildInfo" BeforeTargets="CoreCompile">
+    <PropertyGroup>
+      <BuildInfoFile>$(IntermediateOutputPath)BuildInfo.g.cs</BuildInfoFile>
+    </PropertyGroup>
+    <ItemGroup>
+      <BuildInfoLine Include="namespace Monitor.Protocol%3B" />
+      <BuildInfoLine Include="/// &lt;summary&gt;这个程序是什么时候、从哪个版本构建出来的（构建时生成）。&lt;/summary&gt;" />
+      <BuildInfoLine Include="public static class BuildInfo" />
+      <BuildInfoLine Include="{" />
+      <BuildInfoLine Include="    public const string Version = &quot;$(Version)&quot;%3B" />
+      <BuildInfoLine Include="    public const string Tag = &quot;$(MONITOR_TAG)&quot;%3B" />
+      <BuildInfoLine Include="    public const string Commit = &quot;$(MONITOR_COMMIT)&quot;%3B" />
+      <BuildInfoLine Include="    public const string BuiltAt = &quot;$(MONITOR_BUILT_AT)&quot;%3B" />
+      <BuildInfoLine Include="}" />
+    </ItemGroup>
+    <WriteLinesToFile File="$(BuildInfoFile)" Lines="@(BuildInfoLine)" Overwrite="true" WriteOnlyWhenDifferent="true" />
+    <ItemGroup>
+      <Compile Include="$(BuildInfoFile)" />
+      <FileWrites Include="$(BuildInfoFile)" />
+    </ItemGroup>
+  </Target>
 </Project>