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/cost.py

@@ -4,27 +4,24 @@ Inputs:
     ingredients: list[dict] from recipe.ingredients JSONB
     match_index: dict[ingredient_id, list[match_dict]] — pre-fetched, sorted by confidence DESC
     pantry_ingredient_ids: set of ingredient_ids in home_pantry
+    ingredient_name_index: optional dict[ingredient_id, name_lower] for unit conversion
 
 Strategy:
     For each ingredient in the recipe, take the top-confidence match
-    (or skip if none). Cost = current_price * qty (best-effort scaling
-    that ignores unit conversion — see Limitations below).
-    Savings = max(regular - current, 0) * qty.
-
-Limitations:
-    Unit conversion (lb vs oz, cup vs ml) is NOT implemented in this
-    pass. The qty multiplier is treated as dimensionless. This produces
-    a biased-but-monotonic ranking signal: recipes that use more of an
-    expensive ingredient still rank as more expensive, which is what
-    the planner needs. Real dollar accuracy can come later.
+    (or skip if none). Cost = current_price × converted_qty.
+    Unit conversion is attempted via app.utils.units.convert_qty; if
+    the units are incommensurable, falls back to dimensionless qty to
+    preserve the monotonic ranking signal.
+    Savings = max(regular - current, 0) × converted_qty.
 """
 from __future__ import annotations
 
 from decimal import Decimal
-from typing import Dict, Iterable, List, Set
+from typing import Dict, Iterable, List, Optional, Set
 from uuid import UUID
 
 from app.services.planner.types import IngredientCost, RecipeCost
+from app.utils.units import convert_qty
 
 
 def _decimal(v) -> Decimal:
@@ -87,8 +84,16 @@ def compute_recipe_cost(
         current = _decimal(best.get("current_price"))
         regular = _decimal(best.get("regular_price"))
         is_on_sale = bool(best.get("is_on_sale"))
-        line_cost = _scale(current, qty)
-        line_savings = _scale(max(regular - current, Decimal("0")), qty)
+        converted_qty = float(
+            convert_qty(
+                qty,
+                unit,
+                best.get("grocery_unit"),
+                ingredient_name_lower=best.get("ingredient_name"),
+            )
+        )
+        line_cost = _scale(current, converted_qty)
+        line_savings = _scale(max(regular - current, Decimal("0")), converted_qty)
 
         matched_count += 1
         if is_on_sale: