Handle circular required keys

Use a dummy-value at first and update it later.
Deferrable doesn't seem to work for NOT NULL - only
for FOREIGN KEYs.

postgres/queries/budgets.sql

@@ -1,7 +1,7 @@
 -- name: CreateBudget :one
 INSERT INTO budgets
-(name, last_modification)
-VALUES ($1, NOW())
+(name, income_category_id, last_modification)
+VALUES ($1, $2, NOW())
 RETURNING *;
 
 -- name: SetInflowCategory :exec