fix(planning): replace existing slot in simulation instead of appending

simulateVarietyScore was adding the candidate recipe on top of the
existing slot for slotDate, keeping the old recipe's tag-repeat penalty
in the score. Now the existing slot is excluded before simulating, so
swapping a recipe for one with better variety correctly shows positive
scoreDelta and hasConflict=false.

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

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

@@ -174,7 +174,9 @@ public class PlanningService {
                                          VarietyScoreConfig config, Set<UUID> recentlyCookedIds) {
         List<SimulatedSlot> simulatedSlots = new ArrayList<>();
         for (WeekPlanSlot slot : plan.getSlots()) {
-            simulatedSlots.add(new SimulatedSlot(slot.getRecipe(), slot.getSlotDate()));
+            if (!slot.getSlotDate().equals(slotDate)) {
+                simulatedSlots.add(new SimulatedSlot(slot.getRecipe(), slot.getSlotDate()));
+            }
         }
         simulatedSlots.add(new SimulatedSlot(candidate, slotDate));
         return scoreFromSimulatedSlots(simulatedSlots, config, recentlyCookedIds);

backend/src/test/java/com/recipeapp/planning/SuggestionsTest.java

@@ -329,6 +329,37 @@ class SuggestionsTest {
             assertThat(item.hasConflict()).isTrue();
         }
 
+        @Test
+        void swappingExistingSlotForCleanRecipeShouldHavePositiveDelta() {
+            // Plan has Mon=ItalianA, Tue=ItalianB → consecutive cuisine tag repeat → currentScore = 8.5
+            // Asking for suggestions for Mon (swap scenario).
+            // CleanRecipe (no Italian tag) → correct simulation: [Mon:CleanRecipe, Tue:ItalianB] → no repeat → 10.0
+            // scoreDelta = +1.5 → hasConflict = false
+            var plan = createPlan();
+            var italianTag = createTag("Italienisch", "cuisine");
+            var italianA = createRecipe("Spaghetti Carbonara");
+            addTag(italianA, italianTag);
+            addSlot(plan, italianA, MONDAY);
+            var italianB = createRecipe("Penne Arrabiata");
+            addTag(italianB, italianTag);
+            addSlot(plan, italianB, MONDAY.plusDays(1));
+            var cleanRecipe = createRecipe("Grillhähnchen");
+
+            stubPlan(plan);
+            stubDefaultConfig();
+            stubRecipes(italianA, italianB, cleanRecipe);
+            stubNoCookingLogs();
+
+            SuggestionResponse result = planningService.getSuggestions(
+                    HOUSEHOLD_ID, plan.getId(), MONDAY, List.of(), 5);
+
+            assertThat(result.suggestions()).hasSize(1);
+            var item = result.suggestions().getFirst();
+            assertThat(item.recipe().name()).isEqualTo("Grillhähnchen");
+            assertThat(item.scoreDelta()).isCloseTo(1.5, within(0.001));
+            assertThat(item.hasConflict()).isFalse();
+        }
+
         @Test
         void scoreDeltaIsSortedDescendingCleanBeforeConflicting() {
             // Clean recipe (scoreDelta = 0.0) should rank above conflicting (scoreDelta < 0).