Function start_optical_forgetting_loop 

pub async fn start_optical_forgetting_loop(
    store: Arc<SqliteStore>,
    provider: AnyProvider,
    config: OpticalForgettingConfig,
    forgetting_floor: f32,
    cancel: CancellationToken,
)

Start the background optical forgetting loop.

Periodically scans messages older than the configured thresholds and progressively compresses them. Database errors are logged but do not stop the loop.

The loop respects cancel for graceful shutdown.