web: reset links to the default mode, and keep CN agents dial-only by default [CI]

POST /api/admin/links/reset clears every manual link setting and turns automatic linking on. In the default rules a listening node in CN ranks below a listening node elsewhere, so the CN side dials; the links are pushed again when a node's country is first resolved or changes.

backend/src/MoShiro.Backend.Api/Admin/LinkEndpoints.cs

@@ -21,6 +21,9 @@ public static class LinkEndpoints
         group.MapGet("/links", ListAsync)
             .Api("links_list", "Every possible dialer/listener pair: whether it is linked, the manual override and whether it is connected; plus the nodes and which of them listen", Tag).Returns<LinkMatrixDto>().ProducesProblems(401, 403)
             .Requires(Capability.FleetRead);
+        group.MapPost("/links/reset", ResetAsync)
+            .Api("links_reset", "Drop every manual link setting and turn automatic linking on: who dials whom follows the default rules again", Tag).Returns<LinkMatrixDto>().ProducesProblems(401, 403)
+            .Requires(Capability.FleetWrite);
         group.MapPut("/links/{dialer_id}/{listener_id}", SetAsync)
             .Api("links_set", "Connect a node to a listening node (enabled: true), force the link off (false), or give it back to the automatic setting (null)", Tag).Returns<LinkDto>().ProducesProblems(400, 401, 403, 404, 409, 422)
             .Requires(Capability.FleetWrite);
@@ -29,6 +32,13 @@ public static class LinkEndpoints
             .Requires(Capability.FleetWrite);
     }
 
+    private static async Task<IResult> ResetAsync(AgentService agents, LinkService links, LinkRuntime runtime, TimeProvider time, CancellationToken ct)
+    {
+        await links.ResetAsync(ct);
+        await agents.AfterAgentSetChangedAsync(ct);
+        return await ListAsync(agents, links, runtime, time, ct);
+    }
+
     private static async Task<IResult> ListAsync(AgentService agents, LinkService links, LinkRuntime runtime, TimeProvider time, CancellationToken ct)
     {
         LinkSnapshot snapshot = await links.SnapshotAsync(ct);
@@ -42,8 +52,8 @@ public static class LinkEndpoints
                 (AgentView Dialer, AgentView Listener)? pair =
                     snapshot.IsLinked(nodes[i].Agent, nodes[j].Agent) ? (nodes[i], nodes[j])
                     : snapshot.IsLinked(nodes[j].Agent, nodes[i].Agent) ? (nodes[j], nodes[i])
-                    : LinkSnapshot.IsDefaultDialer(nodes[i].Agent, nodes[j].Agent) ? (nodes[i], nodes[j])
-                    : LinkSnapshot.IsDefaultDialer(nodes[j].Agent, nodes[i].Agent) ? (nodes[j], nodes[i])
+                    : snapshot.IsDefaultDialer(nodes[i].Agent, nodes[j].Agent) ? (nodes[i], nodes[j])
+                    : snapshot.IsDefaultDialer(nodes[j].Agent, nodes[i].Agent) ? (nodes[j], nodes[i])
                     : null;
                 if (pair is { } p)
                 {