Add weekly meal plan shopping list push

[itsDNNS]

Summary

• Add a week-level meal plan action that sends all planned ingredients to a selected shopping list
• Merge duplicate ingredients conservatively when names and units match
• Keep existing single-meal shopping list behavior covered

Test plan

• DATABASE_URL=sqlite:///./test-meal-plans.db JWT_SECRET=test-secret PYTHONPATH=$PWD pytest -q tests/test_meal_plans.py
• npm test -- --runTestsByPath __tests__/components/MealPlansView.test.js --runInBand
• npm run build
• BASE_URL=http://127.0.0.1:3000 npx playwright test e2e/tests/meal-plans.spec.js --project="Desktop Chrome" --reporter=line --timeout=90000 --global-timeout=180000
• BASE_URL=http://127.0.0.1:3000 npx playwright test e2e/tests/meal-plans.spec.js --project="Mobile Chrome" --reporter=line --timeout=90000 --global-timeout=180000

Closes #232