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/planning/PlanningServiceTest.java

@@ -37,7 +37,7 @@ class PlanningServiceTest {
     @Mock private UserAccountRepository userAccountRepository;
     @Mock private VarietyScoreConfigRepository varietyScoreConfigRepository;
 
-    @InjectMocks private PlanningServiceImpl planningService;
+    @InjectMocks private PlanningService planningService;
 
     private static final UUID HOUSEHOLD_ID = UUID.randomUUID();
     private static final LocalDate WEEK_START = LocalDate.of(2026, 4, 6); // Monday