fix: Write-lock individual rooms when building sync for them

src/api/client/sync/v3/joined.rs

@@ -65,6 +65,7 @@ pub(super) async fn load_joined_room(
 	and `join*` functions are used to perform steps in parallel which do not depend on each other.
 	*/
 
+	let insert_lock = services.rooms.timeline.mutex_insert.lock(room_id).await;
 	let (
 		account_data,
 		ephemeral,
@@ -82,6 +83,7 @@ pub(super) async fn load_joined_room(
 	)
 	.boxed()
 	.await?;
+	drop(insert_lock);
 
 	if !timeline.is_empty() || !state_events.is_empty() {
 		trace!(