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/BackendFixture.cs

@@ -264,6 +264,7 @@ public class BackendFixture : IAsyncLifetime
                 options.LoginAttemptsPerMinute = LoginAttemptsPerMinute;
                 options.RequireOwner = false; // 大部分测试不建立信任根；属主必选的测试自己打开（OwnerTests）
                 options.GeoLookupUrl = string.Empty; // 测试里不访问外部服务
+                options.NodeImageRegistry = string.Empty; // 同理：后台不去真实的镜像仓库取清单
                 options.PhaseDurationScale = 0.2; // 端到端测试里 connect、hold 这些固定 30 秒的阶段缩成 6 秒
             },
             configureServices: services => services.AddSingleton<MoShiro.Backend.Application.IReleaseKey>(TestReleaseKey.Instance)); // 登记清单要验发布签名：测试用自己的密钥签