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/Updates/ReleaseService.cs

@@ -100,16 +100,17 @@ public sealed partial class ReleaseService : IDisposable
     /// </summary>
     public async ValueTask<ReleaseOffer?> GetOrFetchAsync(string component, CancellationToken ct)
     {
-        ReleaseOffer? stored = await GetAsync(component, ct);
+        string channel = BackendUpdates.ChannelOf(_options);
+        ReleaseOffer? stored = component == BackendComponent ? await GetBackendAsync(channel, ct) : await GetAsync(component, ct);
         long now = Environment.TickCount64;
-        long wait = stored is null ? 30_000 : 600_000; // 没有就 30 秒试一次，已经有了就隔 10 分钟看一眼有没有更新的
+        long wait = stored is null ? 30_000 : 240_000; // 没有就 30 秒试一次，已经有了就隔 4 分钟看一眼有没有更新的（后台每 5 分钟问一次）
         if (_feedChecked.TryGetValue(component, out long last) && now - last < wait)
         {
             return stored;
         }
 
         _feedChecked[component] = now;
-        if (await _feed.FetchAsync(component, BackendUpdates.ChannelOf(_options), ct) is { } fetched && await _keys.VerifyAsync(fetched.Manifest, fetched.Signature, ct))
+        if (await _feed.FetchAsync(component, channel, ct) is { } fetched && await _keys.VerifyAsync(fetched.Manifest, fetched.Signature, ct))
         {
             Result<ReleaseOffer> published = await PublishAsync(fetched.Manifest, fetched.Signature, ct);
             if (published.IsSuccess)