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/AgentService.cs

@@ -20,8 +20,11 @@ public sealed partial class AgentService
     private readonly TaskPlanner _planner;
     private readonly ISettingsStore _settings;
 
-    public AgentService(IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings)
+    private readonly NodeInfoService _nodes;
+
+    public AgentService(IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings, NodeInfoService nodes)
     {
+        _nodes = nodes;
         _agents = agents;
         _runtime = runtime;
         _gateway = gateway;
@@ -114,6 +117,7 @@ public sealed partial class AgentService
     {
         await _gateway.DisconnectAsync(id, "deleted", ct);
         await _agents.DeleteAsync(id, ct);
+        await _nodes.ForgetAsync(id.Value, ct);
         _runtime.Remove(id);
         await AfterAgentSetChangedAsync(ct);
     }