fix(shopping): NULLIF-Guard gegen servings_default=0 in Aggregation

tests/integration/shopping-repository.test.ts

@@ -172,4 +172,16 @@ describe('listShoppingList aggregation', () => {
     addRecipeToCart(db, id, null);
     expect(listShoppingList(db).uncheckedCount).toBe(2);
   });
+
+  it('does not blow up when servings_default is zero (silent NULL total_quantity)', () => {
+    const db = openInMemoryForTest();
+    const id = insertRecipe(db, recipe({
+      servings_default: 0,
+      ingredients: [{ position: 1, quantity: 100, unit: 'g', name: 'Mehl', note: null, raw_text: '', section_heading: null }]
+    }));
+    addRecipeToCart(db, id, null, 4);
+    const rows = listShoppingList(db).rows;
+    expect(rows).toHaveLength(1);
+    expect(rows[0].total_quantity).toBeNull();
+  });
 });