Function spawn_rag_sync 

pub fn spawn_rag_sync(
    vault: Arc<NoteVault>,
    settings: &SharedSettings,
    tx: AppTx,
) -> Option<JoinHandle<()>>

Spawns the background sync loop for vault if a RAG server is configured. Returns the task handle (abort it when the vault is rebuilt), or None when the feature is off. Status is delivered to the UI via AppEvent::RagStatus.