feat(planner): replace simulatedScore with scoreDelta + hasConflict in SuggestionItem

SuggestionItem now exposes scoreDelta (simulatedScore − currentScore) and
hasConflict (scoreDelta ≤ 0) so the frontend can render badges without
needing to pass currentVarietyScore as a separate prop.

PlanningService.getSuggestions() computes currentScore once per request
and derives scoreDelta + hasConflict per candidate. Sorting is unchanged
(scoreDelta desc = simulatedScore desc since currentScore is constant).

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

backend/src/main/java/com/recipeapp/planning/dto/SuggestionResponse.java

@@ -6,6 +6,7 @@ public record SuggestionResponse(List<SuggestionItem> suggestions) {
 
     public record SuggestionItem(
             SlotResponse.SlotRecipe recipe,
-            double simulatedScore
+            double scoreDelta,
+            boolean hasConflict
     ) {}
 }