feat(planning): add GET /v1/week-plans/{planId}/variety-preview endpoint

Returns currentScore, projectedScore, and scoreDelta when a recipe
would be added on a given date. Used by C6 desktop day picker.

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

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

@@ -96,4 +96,14 @@ public class WeekPlanController {
         UUID householdId = householdResolver.resolve(principal.getName());
         return planningService.getVarietyScore(householdId, id);
     }
+
+    @GetMapping("/{planId}/variety-preview")
+    public VarietyPreviewResponse getVarietyPreview(
+            Principal principal,
+            @PathVariable UUID planId,
+            @RequestParam UUID recipeId,
+            @RequestParam LocalDate date) {
+        UUID householdId = householdResolver.resolve(principal.getName());
+        return planningService.getVarietyPreview(householdId, planId, recipeId, date);
+    }
 }