fix(planning): hasConflict only when scoreDelta strictly negative

Neutral suggestions (scoreDelta = 0) are not conflicts — they simply
don't improve variety. Changing scoreDelta <= 0 to scoreDelta < 0
lets empty-plan additions and quality-neutral swaps show without a
misleading ⚠ Variationskonflikt warning.

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

backend/src/main/java/com/recipeapp/planning/PlanningService.java

@@ -152,7 +152,7 @@ public class PlanningService {
                     double simulatedScore = simulateVarietyScore(
                             plan, candidate, slotDate, config, recentlyCookedIds);
                     double scoreDelta = simulatedScore - currentScore;
-                    boolean hasConflict = scoreDelta <= 0;
+                    boolean hasConflict = scoreDelta < 0;
                     return new SuggestionResponse.SuggestionItem(toSlotRecipe(candidate), scoreDelta, hasConflict);
                 })
                 .sorted((a, b) -> Double.compare(b.scoreDelta(), a.scoreDelta()))