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/config.py

@@ -10,7 +10,7 @@ class PlannerConfig:
     recency_weeks: int = 4              # constraint #3: no repeat within N weeks
     calorie_tolerance_pct: int = 20     # constraint #4: ±X% of family.calorie_target
     max_total_minutes: int = 45         # constraint #5: prep + cook
-    max_meal_cost: float = 30.00        # constraint #6: dollars per meal
+    max_meal_cost: float = 50.00        # constraint #6: dollars per serving (dimensionless proxy — unit conversion not yet implemented)
 
     # Scoring weights (must sum to 1.0)
     w_savings: float = 0.30
@@ -27,8 +27,8 @@ class PlannerConfig:
     recency_full_weeks: int = 12        # full bonus when last cooked >= this many weeks ago
 
     # Set selection
-    top_k: int = 20                     # how many feasible recipes to enumerate over
-    set_size: int = 21                  # 21 meals/week (3 per day × 7 days)
+    top_k: int = 10                     # how many feasible recipes to enumerate over
+    set_size: int = 3                   # 3 dinners/week
     p_protein: float = 0.15             # diversity penalty per shared-protein pair
     p_cuisine: float = 0.10             # diversity penalty per shared-cuisine pair
 

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,
     )

backend/app/services/planner/filter.py

@@ -79,7 +79,7 @@ def filter_recipes(
         if cost is None:
             rejected[rid] = "no_cost"
             continue
-        if float(cost.total_cost) > config.max_meal_cost:
+        if float(cost.cost_per_serving) > config.max_meal_cost:
             rejected[rid] = "cost"
             continue
 

backend/app/services/planner/generate.py

@@ -113,6 +113,7 @@ def generate_meal_plan(
             "protein_type": r.protein_type,
             "cuisine_tags": list(r.cuisine_tags or []),
             "ingredients": list(r.ingredients or []),
+            "servings": r.servings or 4,
         }
         for r in recipes
     ]
@@ -139,6 +140,7 @@ def generate_meal_plan(
             ingredients=r["ingredients"],
             match_index=match_index,
             pantry_ingredient_ids=pantry_ids,
+            servings=r["servings"],
         )
         for r in recipe_dicts
     }
@@ -176,10 +178,10 @@ def generate_meal_plan(
     db.add(plan)
     db.flush()
 
+    _dinner_days = [1, 3, 5]  # Mon, Wed, Fri — spread across the week
     for index, scored_recipe in enumerate(chosen):
-        day = (index % 7) + 1           # 1..7 (Mon..Sun)
-        meal_type_index = index // 7    # 0=breakfast, 1=lunch, 2=dinner
-        meal_type = [MealType.BREAKFAST, MealType.LUNCH, MealType.DINNER][meal_type_index]
+        day = _dinner_days[index] if index < len(_dinner_days) else index + 1
+        meal_type = MealType.DINNER
         item = MealPlanItem(
             meal_plan_id=plan.id,
             recipe_id=scored_recipe.recipe_id,

backend/app/services/planner/select.py

@@ -41,7 +41,8 @@ def select_set(
         return pool, _set_score(pool, config)
 
     pool.sort(key=lambda s: s.score, reverse=True)
-    candidate_pool = pool[: config.top_k]
+    # top_k must cover at least set_size items or combinations() yields nothing
+    candidate_pool = pool[: max(config.top_k, config.set_size)]
 
     best: List[ScoredRecipe] = []
     best_score = float("-inf")

backend/app/services/planner/types.py

@@ -31,8 +31,14 @@ class RecipeCost:
     matched_ingredient_count: int
     total_ingredient_count: int
     pantry_hit_count: int
+    servings: int = 4
     line_items: List[IngredientCost] = field(default_factory=list)
 
+    @property
+    def cost_per_serving(self) -> Decimal:
+        s = max(self.servings, 1)
+        return (self.total_cost / Decimal(s)).quantize(Decimal("0.01"))
+
     @property
     def sale_coverage_pct(self) -> float:
         if self.total_ingredient_count == 0: