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.Application/Agents/AgentService.cs

@@ -21,9 +21,11 @@ public sealed partial class AgentService
     private readonly ISettingsStore _settings;
 
     private readonly NodeInfoService _nodes;
+    private readonly LinkService _links;
 
-    public AgentService(IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings, NodeInfoService nodes)
+    public AgentService(IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings, NodeInfoService nodes, LinkService links)
     {
+        _links = links;
         _nodes = nodes;
         _agents = agents;
         _runtime = runtime;
@@ -118,17 +120,19 @@ public sealed partial class AgentService
         await _gateway.DisconnectAsync(id, "deleted", ct);
         await _agents.DeleteAsync(id, ct);
         await _nodes.ForgetAsync(id.Value, ct);
+        await _links.ForgetNodeAsync(id.Value, ct);
         _runtime.Remove(id);
         await AfterAgentSetChangedAsync(ct);
     }
 
-    /// <summary>Probe 需要连接的 Agent 列表（所有已激活的 Agent）。版本号在列表变化时递增。</summary>
-    public async ValueTask<ProbeAgentList> ProbeAgentsAsync(CancellationToken ct)
+    /// <summary>这个 Probe 要连接的 Agent 列表（已激活、有公开地址，并且线路是开着的）。版本号在列表变化时递增。</summary>
+    public async ValueTask<ProbeAgentList> ProbeAgentsAsync(string probeId, CancellationToken ct)
     {
+        LinkSnapshot links = await _links.SnapshotAsync(ct);
         IReadOnlyList<AgentRecord> agents = await _agents.ListAsync(ct);
         long version = long.TryParse(await _settings.GetAsync(TaskPlanner.ProbeAgentsVersionKey, ct), out long v) ? v : 0;
         var result = new ProbeAgentList { Version = version };
-        foreach (AgentRecord agent in agents.Where(a => a.Status == NodeStatus.Active && a.PublicHost.Length > 0))
+        foreach (AgentRecord agent in agents.Where(a => a.Status == NodeStatus.Active && a.PublicHost.Length > 0 && links.IsLinked(probeId, a.Id.Value)))
         {
             result.Agents.Add(new AgentEndpoint
             {