feat(ingredients): make units nullable

migrations/000086_make_ingredients_unit_id_nullable.up.sql

@@ -0,0 +1,5 @@
+-- An ingredient can be registered before its unit has been decided, so unit_id
+-- is optional. Existing rows are untouched: they already have a unit.
+ALTER TABLE ingredients ALTER COLUMN unit_id DROP NOT NULL;
+
+COMMENT ON COLUMN ingredients.unit_id IS 'Base unit of the ingredient. NULL means no unit has been assigned yet.';