Implement Recipe, Planning, Shopping, Pantry, and Admin domains

Outside-in TDD for all 5 remaining domains (128 tests total):
- Recipe: CRUD, ingredients autocomplete/patch, tags, categories (27 tests)
- Planning: week plans, slots, confirm, suggestions, variety score, cooking logs (24 tests)
- Shopping: generate from plan, publish, check/add/remove items (15 tests)
- Pantry: CRUD with expiry sorting (11 tests)
- Admin: user management, password reset, audit logging (13 tests)

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

backend/src/main/java/com/recipeapp/pantry/PantryService.java

@@ -0,0 +1,17 @@
+package com.recipeapp.pantry;
+
+import com.recipeapp.pantry.dto.*;
+
+import java.util.List;
+import java.util.UUID;
+
+public interface PantryService {
+
+    List<PantryItemResponse> listItems(UUID householdId);
+
+    PantryItemResponse createItem(UUID householdId, CreatePantryItemRequest request);
+
+    PantryItemResponse updateItem(UUID householdId, UUID itemId, UpdatePantryItemRequest request);
+
+    void deleteItem(UUID householdId, UUID itemId);
+}