fix(planner): correct set_size to 3 dinners and switch cost filter to per-serving

- set_size 21→3, top_k 20→10: generate 3 weekly dinners not full 3×7 matrix
- All 3 items assigned MealType.DINNER on Mon/Wed/Fri
- RecipeCost gains servings field + cost_per_serving property
- compute_recipe_cost accepts servings param (default 4)
- filter.py gates on cost_per_serving instead of total_cost
- max_meal_cost 500→50 (now a meaningful $/serving threshold)
- Email displays ~$X/serving instead of inflated raw total
- select_set: candidate_pool uses max(top_k, set_size) to prevent
  combinations(n<set_size) returning empty iterator

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/planner/cost.py

@@ -43,6 +43,7 @@ def compute_recipe_cost(
     ingredients: Iterable[dict],
     match_index: Dict[UUID, List[dict]],
     pantry_ingredient_ids: Set[UUID],
+    servings: int = 4,
 ) -> RecipeCost:
     line_items: List[IngredientCost] = []
     total_cost = Decimal("0.00")
@@ -119,5 +120,6 @@ def compute_recipe_cost(
         matched_ingredient_count=matched_count,
         total_ingredient_count=total,
         pantry_hit_count=pantry_hits,
+        servings=max(servings, 1),
         line_items=line_items,
     )