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/tests/Monitor.Measurement.Tests/PlatformDetectorTests.cs

@@ -0,0 +1,27 @@
+namespace Monitor.Measurement.Tests;
+
+public sealed class PlatformDetectorTests
+{
+    [Theory]
+    [InlineData("PRETTY_NAME=\"Debian GNU/Linux 12 (bookworm)\"|NAME=\"Debian GNU/Linux\"|VERSION_ID=\"12\"|ID=debian", "Debian 12")]
+    [InlineData("NAME=\"OpenWrt\"|VERSION_ID=\"24.10.0\"|ID=openwrt", "OpenWrt 24.10")]
+    [InlineData("NAME=\"Alpine Linux\"|ID=alpine|VERSION_ID=3.22.1", "Alpine Linux 3.22")]
+    [InlineData("NAME=\"Ubuntu\"|VERSION_ID=\"24.04\"", "Ubuntu 24.04")]
+    [InlineData("ID=arch", "arch")]
+    [InlineData("", "Linux")]
+    public void LinuxName_IsTheDistroAndItsMajorMinorVersion(string osRelease, string expected)
+    {
+        Assert.Equal(expected, PlatformDetector.LinuxName(osRelease.Split('|', StringSplitOptions.RemoveEmptyEntries)));
+    }
+
+    [Fact]
+    public void Detect_ReportsOnlyCoarseInformation()
+    {
+        var platform = PlatformDetector.Detect();
+        Assert.Contains(platform.Os, new[] { "linux", "windows", "macos", "other" });
+        Assert.Contains(platform.Arch, new[] { "x64", "arm64", "arm", "x86" });
+        // 没有主机名、内核版本：名字很短，也不含小数点以外的版本细节
+        Assert.True((platform.Name ?? string.Empty).Length < 40);
+        Assert.DoesNotContain(Environment.MachineName, platform.Name ?? string.Empty, StringComparison.OrdinalIgnoreCase);
+    }
+}