Use new UpdateAssignmentWithDifference for ynab-import to prevent errors on duplicates

postgres/queries/assignments.sql

@@ -29,4 +29,11 @@ INSERT INTO assignments (category_id, date, amount)
 VALUES($1, $2, $3) 
 ON CONFLICT (category_id, date) 
 DO 
-   UPDATE SET amount = $3;
+   UPDATE SET amount = $3;
+
+-- name: UpdateAssignmentWithDifference :exec
+INSERT INTO assignments (category_id, date, amount)
+VALUES($1, $2, $3) 
+ON CONFLICT (category_id, date) 
+DO 
+   UPDATE SET amount = assignments.amount + $3;