feat: choose which probes link to which agents from the admin

The panel decides the probe/agent pairs and probes always dial out. A new
fleet.auto_link setting connects every probe to every agent (default); with it
off only the pairs chosen by hand connect, and any pair can be forced on or
off. The Probes page has a link matrix with the switch and per-pair toggles.

backend/src/Monitor.Backend.Api/Contracts/ApiJsonContext.cs

@@ -37,6 +37,9 @@ namespace Monitor.Backend.Api;
 [JsonSerializable(typeof(AgentDto))]
 [JsonSerializable(typeof(PageDto<AgentDto>))]
 [JsonSerializable(typeof(AgentPatchRequest))]
+[JsonSerializable(typeof(LinkDto))]
+[JsonSerializable(typeof(LinkMatrixDto))]
+[JsonSerializable(typeof(SetLinkRequest))]
 [JsonSerializable(typeof(BuildDto))]
 [JsonSerializable(typeof(ReleaseDto))]
 [JsonSerializable(typeof(PageDto<ReleaseDto>))]