Batch 6: Silent polling + SignalR Hub download source

[JusterZhu]

Batch 6: Silent Polling and SignalR Hub Integration

Tasks

1. SilentPollOrchestrator — Timed polling orchestrator with configurable interval, background download, optional auto-install

2. HubDownloadSource — SignalR Hub download source implementing IDownloadSource

3. Silent mode integration — Wire Silent=true in GeneralUpdateBootstrap to start SilentPollOrchestrator, consume SilentPollInterval/SilentAutoInstall UpdateOptions

4. SilentUpdateMode old class cleanup — Remove or mark obsolete after migration

Related

• Refactor plan: GeneralUpdate-refactor-plan.md