fix: the panel polls the registry for its own and the nodes' releases [CI]

Registering a manifest with the panel is best effort in CI (an unreachable
panel only warns, the pipeline stays green). The Backend update notice then
never appeared, because the host updater only reads the mailbox the panel
writes. A background poller now fetches the signed manifests of the agent, the
probe and the backend itself from the registry every five minutes, verifies the
release signature and registers them, writing the backend one into the host
updater's mailbox.

backend/tests/MoShiro.Backend.Tests/ReleaseFeedTests.cs

@@ -19,6 +19,8 @@ public sealed class ReleaseFeedTests : IAsyncLifetime
     private WebApplication _registry = null!;
     private string _registryUrl = "";
     private string _label = "";
+    private string _backendLabel = "";
+    private string _backendSignature = "";
 
     public async ValueTask InitializeAsync()
     {
@@ -28,6 +30,10 @@ public sealed class ReleaseFeedTests : IAsyncLifetime
         _registry.MapGet("/v2/moshiro-dev/moshiro-agent-manifest/manifests/dev", () => Results.Text(new JsonObject { ["schemaVersion"] = 2, ["config"] = new JsonObject { ["digest"] = ConfigDigest } }.ToJsonString(), "application/vnd.oci.image.manifest.v1+json"));
         _registry.MapGet("/v2/moshiro-dev/moshiro-agent-manifest/blobs/" + ConfigDigest, () => Results.Text(
             new JsonObject { ["config"] = new JsonObject { ["Labels"] = new JsonObject { ["org.moshiro.manifest"] = _label, ["org.moshiro.signature"] = "c2ln" } } }.ToJsonString(), "application/json"));
+        const string BackendConfigDigest = "sha256:0000000000000000000000000000000000000000000000000000000000000002";
+        _registry.MapGet("/v2/moshiro-dev/moshiro-backend-manifest/manifests/dev", () => Results.Text(new JsonObject { ["schemaVersion"] = 2, ["config"] = new JsonObject { ["digest"] = BackendConfigDigest } }.ToJsonString(), "application/vnd.oci.image.manifest.v1+json"));
+        _registry.MapGet("/v2/moshiro-dev/moshiro-backend-manifest/blobs/" + BackendConfigDigest, () => Results.Text(
+            new JsonObject { ["config"] = new JsonObject { ["Labels"] = new JsonObject { ["org.moshiro.manifest"] = _backendLabel, ["org.moshiro.signature"] = _backendSignature } } }.ToJsonString(), "application/json"));
         await _registry.StartAsync();
         _registryUrl = _registry.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()!.Addresses.First();
     }
@@ -71,6 +77,56 @@ public sealed class ReleaseFeedTests : IAsyncLifetime
         Assert.Null(await empty.FetchAsync("agent", "dev", default));
     }
 
+    [Fact]
+    public async Task ThePanelFindsItsOwnNewRelease_InTheRegistry_AndLeavesItForTheHostUpdater()
+    {
+        // CI 把清单登记给面板那一步失败了（只警告）：面板自己去仓库取，验过签就登记，并写进宿主机更新器的信箱
+        long seq = TimeProvider.System.GetUtcNow().ToUnixTimeSeconds();
+        string manifest = $$"""{"schema":1,"component":"backend","channel":"dev","image":"registry.example/moshiro-dev/moshiro-backend","digest":"sha256:2222222222222222222222222222222222222222222222222222222222222222","tag":"dev-new","version":"0.1.1","seq":{{seq}}}""";
+        _backendLabel = Convert.ToBase64String(Encoding.UTF8.GetBytes(manifest));
+        _backendSignature = TestReleaseKey.Sign(manifest);
+        string directory = Path.Combine(Path.GetTempPath(), "moshiro-feed-" + Guid.NewGuid().ToString("N"));
+        await using WebApplication panel = BackendHost.Build(
+            ["--urls", "http://127.0.0.1:0", "--Logging:LogLevel:Default=Warning"],
+            options =>
+            {
+                options.DataDirectory = directory;
+                options.Pbkdf2Iterations = 1000;
+                options.SecureCookies = false;
+                options.NodeImageRegistry = _registryUrl;
+                options.UpdateChannel = "dev";
+                options.GeoLookupUrl = string.Empty;
+                options.RequireOwner = false;
+            },
+            configureServices: services => services.AddSingleton<IReleaseKey>(TestReleaseKey.Instance));
+        await panel.StartAsync();
+        try
+        {
+            ReleaseService releases = panel.Services.GetRequiredService<ReleaseService>();
+            Assert.Null(await releases.GetBackendAsync("dev", default));
+
+            ReleaseOffer? offer = await releases.GetOrFetchAsync("backend", default);
+
+            Assert.Equal(seq, offer!.Seq);
+            Assert.Equal(seq, (await releases.GetBackendAsync("dev", default))!.Seq);
+            Assert.Equal(manifest, await File.ReadAllTextAsync(Path.Combine(directory, "update", "offer.manifest")));
+        }
+        finally
+        {
+            await panel.StopAsync();
+            await panel.DisposeAsync();
+            Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+            try
+            {
+                Directory.Delete(directory, recursive: true);
+            }
+            catch (IOException)
+            {
+                // 系统临时目录里的残留
+            }
+        }
+    }
+
     [Fact]
     public async Task AForgedManifestInTheRegistry_IsNeverServedToTheInstaller()
     {