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/orchestrator/steps.py

@@ -23,6 +23,7 @@ from app.services.email import get_email_backend
 from app.services.orchestrator.alerts import send_admin_alert
 from app.services.planner.generate import generate_meal_plan
 from app.services.scraper_service import ScraperService
+from app.utils.units import convert_qty
 
 from app.services.feedback_analyzer import FeedbackAnalyzer
 from app.services.recipe_discovery import RecipeDiscoveryService
@@ -254,7 +255,16 @@ def step_email(run: "WeeklyRun", db: "Session") -> None:
                     .first()
                 )
                 if match and match.grocery_item and match.grocery_item.current_price:
-                    est_cost_total += float(match.grocery_item.current_price)
+                    qty = ing.get("qty", 1.0)
+                    unit = ing.get("unit", "")
+                    gunit = match.grocery_item.unit or ""
+                    converted = convert_qty(
+                        qty,
+                        unit,
+                        gunit,
+                        ingredient_name_lower=ing_name,
+                    )
+                    est_cost_total += float(match.grocery_item.current_price) * float(converted)
 
             recipe_servings = (item.recipe.servings or 4) if item.recipe else 4
             est_cost_per_serving = est_cost_total / recipe_servings
@@ -408,10 +418,20 @@ def step_finalize(run: "WeeklyRun", db: "Session") -> None:
                     .first()
                 )
                 if match and match.grocery_item:
-                    lucky_name = html.escape(match.grocery_item.name or "")
-                    price = float(match.grocery_item.current_price or 0)
-                    total_cost += price
-                    price_str = f"${price:.2f}"
+                    g_item = match.grocery_item
+                    lucky_name = html.escape(g_item.name or "")
+                    price = float(g_item.current_price or 0)
+                    qty = ing.get("qty", 1.0)
+                    unit = ing.get("unit", "")
+                    gunit = g_item.unit or ""
+                    converted = convert_qty(
+                        qty,
+                        unit,
+                        gunit,
+                        ingredient_name_lower=ing_name,
+                    )
+                    total_cost += price * float(converted)
+                    price_str = f"${price * float(converted):.2f}"
                 else:
                     lucky_name = "—"
                     price_str = "—"