Remove service interfaces — use concrete classes directly

Each domain had a single-implementation interface (e.g. AdminService
interface + AdminServiceImpl). Merged implementation into the service
class and deleted the redundant interfaces per KISS principle.

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

backend/src/test/java/com/recipeapp/recipe/RecipeServiceTest.java

@@ -28,7 +28,7 @@ class RecipeServiceTest {
     @Mock private IngredientCategoryRepository ingredientCategoryRepository;
     @Mock private HouseholdRepository householdRepository;
 
-    @InjectMocks private RecipeServiceImpl recipeService;
+    @InjectMocks private RecipeService recipeService;
 
     private static final UUID HOUSEHOLD_ID = UUID.randomUUID();