feat(backend): implement unit conversion for cost calculation

- Add UnitConverter (normalization, within-family, density tables)
- Update cost.py to convert recipe qty to grocery price unit
- Update generate.py _load_match_index to fetch ingredient name + unit
- Fix orchestrator email/shopping-list cost loops to use conversion
- Fix missing Ingredient import in generate.py
- Add 19 unit tests

backend/tests/test_planner_cost.py

@@ -15,10 +15,12 @@ def _match(grocery_id, name, current, regular, is_on_sale, confidence=0.9):
     return {
         "grocery_item_id": grocery_id,
         "grocery_item_name": name,
+        "ingredient_name": name.lower(),
         "current_price": Decimal(str(current)),
         "regular_price": Decimal(str(regular)),
         "is_on_sale": is_on_sale,
         "confidence": Decimal(str(confidence)),
+        "grocery_unit": None,
     }