feat(shopping): add getByWeekStart to ShoppingService

Returns the shopping list for a given week, defaulting to the current
week's Monday when no weekStart is provided. Returns null when no
list exists.

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

backend/src/main/java/com/recipeapp/shopping/ShoppingService.java

@@ -18,10 +18,11 @@ import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
 import java.math.BigDecimal;
+import java.time.DayOfWeek;
+import java.time.LocalDate;
+import java.time.temporal.TemporalAdjusters;
 import java.util.*;
 import java.util.stream.Collectors;
-import java.util.Set;
-import java.util.Map;
 
 @Service
 @Transactional
@@ -52,6 +53,18 @@ public class ShoppingService {
     }
 
 
+    @Transactional(readOnly = true)
+    public ShoppingListResponse getByWeekStart(UUID householdId, LocalDate weekStart) {
+        if (weekStart == null) {
+            weekStart = LocalDate.now().with(TemporalAdjusters.previousOrSame(DayOfWeek.MONDAY));
+        }
+
+        return shoppingListRepository.findByHouseholdIdAndWeekPlanWeekStart(householdId, weekStart)
+                .map(this::toResponse)
+                .orElse(null);
+    }
+
+
     public ShoppingListResponse generateFromPlan(UUID householdId, UUID weekPlanId) {
         WeekPlan weekPlan = weekPlanRepository.findById(weekPlanId)
                 .orElseThrow(() -> new ResourceNotFoundException("Week plan not found"));