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/src/MoShiro.Backend.Application/ApplicationServiceCollectionExtensions.cs

@@ -71,6 +71,7 @@ public static class ApplicationServiceCollectionExtensions
         services.AddHostedService(provider => provider.GetRequiredService<TestFinalizer>());
         services.AddHostedService<HotLayerBackfill>();
         services.AddHostedService<RetentionService>();
+        services.AddHostedService<ReleasePoller>();
         return services;
     }
 }