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/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);