Function enqueue_to_device 

pub fn enqueue_to_device(
    conn: &mut Connection,
    sender_user_id: i64,
    messages: &[(i64, String, String, String)],
) -> Result<i64>

Fan out one PUT /sendToDevice call’s messages map (already flattened to one row per (recipient_user_id, recipient_device_id) — a * device wildcard is resolved by the caller before this function ever sees it) as one transaction sharing matrix_store’s global stream counter. Returns the last stream id minted, or the counter’s current value unchanged if messages is empty.