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