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.Application/Agents/AgentEventService.cs

@@ -9,12 +9,14 @@ public sealed class AgentEventService
     private readonly AgentRuntime _runtime;
     private readonly IEventStore _events;
     private readonly TimeProvider _time;
+    private readonly NodeInfoService _nodes;
 
-    public AgentEventService(AgentRuntime runtime, IEventStore events, TimeProvider time)
+    public AgentEventService(AgentRuntime runtime, IEventStore events, TimeProvider time, NodeInfoService nodes)
     {
         _runtime = runtime;
         _events = events;
         _time = time;
+        _nodes = nodes;
     }
 
     public async ValueTask OnEventAsync(NodeId agent, AgentEventParams e, CancellationToken ct)
@@ -32,7 +34,8 @@ public sealed class AgentEventService
         switch (e.Kind)
         {
             case AgentEventKinds.ProbeConnected when !string.IsNullOrEmpty(e.Detail):
-                state.AddProbe(e.Probe is { } p ? new ConnectedProbeInfo(e.Detail, p.Version, p.Remote, p.SinceUtcMs > 0 ? p.SinceUtcMs : at) : new ConnectedProbeInfo(e.Detail, null, null, at));
+                state.AddProbe(e.Probe is { } p ? new ConnectedProbeInfo(e.Detail, p.Version, p.Remote, p.SinceUtcMs > 0 ? p.SinceUtcMs : at, p.Platform) : new ConnectedProbeInfo(e.Detail, null, null, at));
+                _nodes.ObserveProbeInBackground(e.Detail, e.Probe?.Remote, e.Probe?.Platform);
                 break;
             case AgentEventKinds.ProbeDisconnected when !string.IsNullOrEmpty(e.Detail):
                 state.RemoveProbe(e.Detail);