fix: Send pre-message after successful sending of post-message (#8063)

On the receiving side, download post messages w/o known pre-message after 30s to protect from lost
messages.

This isn't easy to test in Rust though because the `smtp` sending code requires an SMTP connection,
so it's only tested (by already existing tests) that messages are queued in the right order.

Another problem is that the `smtp` sending logic doesn't try to send any messages following a
failed-to-send message until the retry limit is reached, so if there's smth wrong with a
post-message, other unrelated messages are delayed, but this problem has already existed.

src/smtp.rs

@@ -379,7 +379,7 @@ pub(crate) async fn send_msg_to_smtp(
     if retries > 6 {
         context
             .sql
-            .execute("DELETE FROM smtp WHERE id=?", (rowid,))
+            .execute("DELETE FROM smtp WHERE msg_id=?", (msg_id,))
             .await
             .context("Failed to remove message with exceeded retry limit from smtp table")?;
         if let Some(mut msg) = Message::load_from_db_optional(context, msg_id).await? {