Implement backend

postgres/queries/categories.sql

@@ -8,6 +8,11 @@ RETURNING *;
 SELECT category_groups.* FROM category_groups 
 WHERE category_groups.budget_id = $1;
 
+-- name: GetCategoryGroupByName :one
+SELECT category_groups.* FROM category_groups 
+WHERE category_groups.budget_id = $1
+AND category_groups.name = $2;
+
 -- name: CreateCategory :one
 INSERT INTO categories
 (name, category_group_id)