feat: choose probe/agent links from the admin, fix the overview popover [CI]

The panel decides which probes link to which agents (probes dial out): an
auto-link setting plus per-pair manual overrides with a link matrix in the
admin, and the hovered overview card is lifted above its neighbours.

backend/src/Monitor.Backend.Application/ApplicationServiceCollectionExtensions.cs

@@ -29,6 +29,7 @@ public static class ApplicationServiceCollectionExtensions
         services.AddSingleton<HostStatusService>();
         services.AddSingleton<NodeInfoService>();
         services.AddSingleton<ProbeService>();
+        services.AddSingleton<LinkService>();
         services.AddSingleton<LineService>();
         services.AddSingleton<TestService>();
         services.AddSingleton<IngestService>();