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/Admin/LinkEndpoints.cs

@@ -0,0 +1,59 @@
+using Fleet.Core;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Routing;
+using Monitor.Backend.Application;
+using Monitor.Protocol;
+
+namespace Monitor.Backend.Api;
+
+/// <summary>线路开关：哪些（Probe，Agent）要连。自动连接的总开关是设置 <c>fleet.auto_link</c>；这里看每一对的结果，并且手动强制开、关或取消手动设置。</summary>
+public static class LinkEndpoints
+{
+    private const string Tag = "admin-links";
+
+    public static void Map(RouteGroupBuilder group)
+    {
+        group.MapGet("/links", ListAsync)
+            .Api("links_list", "Every probe/agent pair: whether it is linked, the manual override and whether it is connected", Tag).Returns<LinkMatrixDto>().ProducesProblems(401, 403);
+        group.MapPut("/links/{probe_id}/{agent_id}", SetAsync)
+            .Api("links_set", "Force a link on or off, or give it back to the automatic setting (enabled: null)", Tag).Returns<LinkDto>().ProducesProblems(400, 401, 403, 404);
+    }
+
+    private static async Task<IResult> ListAsync(AgentService agents, ProbeService probes, LinkService links, CancellationToken ct)
+    {
+        LinkSnapshot snapshot = await links.SnapshotAsync(ct);
+        IReadOnlyList<AgentView> agentViews = await agents.ListAsync(ct);
+        var items = new List<LinkDto>();
+        foreach (ProbeView probe in (await probes.ListAsync(ct)).Where(p => !p.Revoked).OrderBy(p => p.Probe.Name, StringComparer.Ordinal))
+        {
+            foreach (AgentView agent in agentViews.OrderBy(a => a.Agent.Name, StringComparer.Ordinal))
+            {
+                items.Add(ToDto(snapshot, probe, agent));
+            }
+        }
+
+        return TypedResults.Json(new LinkMatrixDto(snapshot.Auto, items), ApiJsonContext.Default.LinkMatrixDto);
+    }
+
+    private static async Task<IResult> SetAsync(HttpContext http, AgentService agents, ProbeService probes, LinkService links, string probe_id, string agent_id, SetLinkRequest request, CancellationToken ct)
+    {
+        if (await probes.GetAsync(probe_id, ct) is not { Revoked: false } probe)
+        {
+            return Problems.Create(http, MonitorErrors.ProbeNotFound, "probe not found");
+        }
+
+        if (await agents.GetAsync(new NodeId(agent_id), ct) is not { } agent)
+        {
+            return Problems.Create(http, MonitorErrors.AgentNotFound, "agent not found");
+        }
+
+        await links.SetAsync(probe_id, agent_id, request.Enabled, ct);
+        await agents.AfterAgentSetChangedAsync(ct); // 重新下发：Probe 的 Agent 列表、Agent 允许的 Probe
+        return TypedResults.Json(ToDto(await links.SnapshotAsync(ct), probe, agent), ApiJsonContext.Default.LinkDto);
+    }
+
+    private static LinkDto ToDto(LinkSnapshot snapshot, ProbeView probe, AgentView agent) =>
+        new(probe.Probe.Id, probe.Probe.Name, agent.Agent.Id.Value, agent.Agent.Name, snapshot.IsLinked(probe.Probe.Id, agent.Agent.Id.Value), snapshot.OverrideOf(probe.Probe.Id, agent.Agent.Id.Value),
+            agent.Probes.Any(p => p.Id == probe.Probe.Id));
+}

backend/src/Monitor.Backend.Api/Admin/SystemEndpoints.cs

@@ -63,9 +63,14 @@ public static class SystemEndpoints
     private static async Task<IResult> ListSettingsAsync(SettingsService settings, CancellationToken ct) =>
         TypedResults.Json(new PageDto<SettingDto>([.. (await settings.ListAsync(ct)).Select(Mappers.ToDto)], null), ApiJsonContext.Default.PageDtoSettingDto);
 
-    private static async Task<IResult> PatchSettingsAsync(HttpContext http, SettingsService settings, Dictionary<string, JsonElement> changes)
+    private static async Task<IResult> PatchSettingsAsync(HttpContext http, SettingsService settings, AgentService agents, Dictionary<string, JsonElement> changes)
     {
         Result<IReadOnlyList<SettingView>> result = await settings.PatchAsync(changes, http.RequestAborted);
+        if (result.IsSuccess && changes.ContainsKey(SettingsService.AutoLink))
+        {
+            await agents.AfterAgentSetChangedAsync(http.RequestAborted); // 自动连接开关变了：重新下发各 Probe 的 Agent 列表和各 Agent 允许的 Probe
+        }
+
         return result.TryGet(out IReadOnlyList<SettingView>? views, out FleetError? error)
             ? TypedResults.Json(new PageDto<SettingDto>([.. views.Select(Mappers.ToDto)], null), ApiJsonContext.Default.PageDtoSettingDto)
             : Problems.From(http, error);