Move idle compaction request creation to backend

[ethanndickson]

Right now, the backend cannot construct compaction requests because it lacks access to user preferences stored in the frontend's local storage. These include:

• Preferred compaction model
• Gateway routing
• Preferred compaction thinking level

This means the idle compaction flow involves the backend sending a 'perform compaction' event to the frontend. This means a standalone mux server instance cannot perform idle compaction.

Once these preferences are persisted in a backend-accessible storage (probably a config file), and served over oRPC, the entire idle compaction flow can happen on the backend. Removing useIdleCompactionHandler hook, and having IdleCompactionService directly call sendMessage.

• Persist compaction preferences to backend-accessible config
• Create compaction request on backend