feat(ingredients): make units nullable

migrations/000086_make_ingredients_unit_id_nullable.down.sql

@@ -0,0 +1,6 @@
+-- Restoring NOT NULL fails if any ingredient still has a NULL unit_id. Assign a
+-- unit to those rows first:
+--   SELECT id, name FROM ingredients WHERE unit_id IS NULL;
+COMMENT ON COLUMN ingredients.unit_id IS NULL;
+
+ALTER TABLE ingredients ALTER COLUMN unit_id SET NOT NULL;