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/app/services/planner/generate.py

@@ -12,6 +12,7 @@ from app.models import (
     FamilyProfile,
     GroceryItem,
     HomePantry,
+    Ingredient,
     IngredientGroceryMatch,
     MealPlan,
     MealPlanItem,
@@ -32,13 +33,14 @@ from app.services.planner.types import GenerationResult
 def _load_match_index(db: Session) -> Dict[UUID, List[dict]]:
     """ingredient_id → list of match dicts ordered by confidence DESC."""
     rows = (
-        db.query(IngredientGroceryMatch, GroceryItem)
+        db.query(IngredientGroceryMatch, GroceryItem, Ingredient)
         .join(GroceryItem, GroceryItem.id == IngredientGroceryMatch.grocery_item_id)
+        .join(Ingredient, Ingredient.id == IngredientGroceryMatch.ingredient_id)
         .order_by(IngredientGroceryMatch.confidence.desc())
         .all()
     )
     index: Dict[UUID, List[dict]] = {}
-    for match, grocery in rows:
+    for match, grocery, ingredient in rows:
         index.setdefault(match.ingredient_id, []).append(
             {
                 "grocery_item_id": grocery.id,
@@ -47,6 +49,8 @@ def _load_match_index(db: Session) -> Dict[UUID, List[dict]]:
                 "regular_price": grocery.regular_price,
                 "is_on_sale": bool(grocery.is_on_sale),
                 "confidence": match.confidence,
+                "ingredient_name": ingredient.name.lower(),
+                "grocery_unit": grocery.unit,
             }
         )
     return index