Add category.order to reorder siblings under a parent.

Sets each direct child's sort to its index in the given categoryIds,
scoped by parentId. Validates the set matches the current children.

src/sql/category.js

@@ -91,6 +91,18 @@ FROM category
 WHERE parent_id = @parentId
 `;
 
+export const LIST_CHILD_IDS = `
+SELECT id
+FROM category
+WHERE parent_id = @parentId
+`;
+
+export const UPDATE_SORT = `
+UPDATE category
+SET sort = @sort
+WHERE id = @id AND parent_id = @parentId
+`;
+
 export const DELETE = `
 DELETE FROM category
 WHERE id = @id