fix(17): WR-02 keep one reorder save in flight and ignore stale responses

A move made while a save runs is queued and sent after it settles, so
out-of-order completion can no longer leave the database on an older tree
than the screen. A failure that a waiting save supersedes keeps the later
moves, and a response tied to an earlier load or partition leaves the
current tree and its last saved state alone.

docs/backend/lists-and-filters.md

@@ -205,7 +205,7 @@ A save posts `{partition, nodes: [{id, parent_id, sort_order}]}` to `POST .../re
 
 Both routes need the controller's permissions; the save is a write route (CSRF header, and a personal token needs a scope that allows POST).
 
-In the admin, a list whose schema carries `reorder` shows a "Reorder records" button before "New record". It opens the reorder page, and when the list is filtered on the partition column it opens that partition. The page shows the tree with a drag handle and move up, move down, outdent and indent buttons on every row. A row can also be dragged before, after or into another row (never into its own subtree), and the arrow keys on a focused handle move it the same way as the buttons. There is no Save button: 400 ms after the last move the page posts the whole set, shows "Order saved", and sends a pending save at once when the administrator leaves. A refused save puts the last saved order back; a 409 reloads the tree. A partitioned page has a selector for the partition.
+In the admin, a list whose schema carries `reorder` shows a "Reorder records" button before "New record". It opens the reorder page, and when the list is filtered on the partition column it opens that partition. The page shows the tree with a drag handle and move up, move down, outdent and indent buttons on every row. A row can also be dragged before, after or into another row (never into its own subtree), and the arrow keys on a focused handle move it the same way as the buttons. There is no Save button: 400 ms after the last move the page posts the whole set, shows "Order saved", and sends a pending save at once when the administrator leaves. Only one save is in flight at a time: moves made while it runs are sent after it settles, with the tree as it is then, so the stored order never ends older than the screen. A refused save puts the last saved order back, unless a later save is waiting (it carries the whole tree); a 409 reloads the tree. A save that settles after the page switched partition does not touch the new partition's tree. A partitioned page has a selector for the partition.
 
 ## Remembered list state