feat(meals): enforce three lower-calorie dinners with media

backend/app/services/planner/config.py

@@ -37,6 +37,8 @@ class PlannerConfig:
         total = self.w_savings + self.w_coverage + self.w_pantry + self.w_time + self.w_recency
         if abs(total - 1.0) > 1e-6:
             raise ValueError(f"weights must sum to 1.0, got {total}")
+        if self.set_size != 3:
+            raise ValueError("set_size must be 3 dinners per week")
 
     def merge(self, overrides: "dict[str, Any]") -> "PlannerConfig":
         """Return a new PlannerConfig with overrides applied."""