Function drain_pending_favorites 

pub async fn drain_pending_favorites(
    db: &Arc<RusqliteService>,
    sink: &dyn FavoriteSink,
    user_id: &str,
) -> Result<Vec<String>, String>

Push pending favourite changes to the server and clear their flags.

Returns the ids that reached the server, for the favorites-changed event. A row whose push fails keeps pending_sync = 1 and is retried on the next reconnect rather than being dropped.

TRACES: UR-069 | DR-120 | UT-103