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/household/HouseholdServiceTest.java

@@ -40,7 +40,7 @@ class HouseholdServiceTest {
     @Mock private VarietyScoreConfigRepository varietyScoreConfigRepository;
 
     @InjectMocks
-    private HouseholdServiceImpl householdService;
+    private HouseholdService householdService;
 
     private UserAccount testUser() {
         return new UserAccount("sarah@example.com", "Sarah", "hashed");