fix: resolve all P0/P1 code quality issues

P0 fixes:
- P0-01: sticky_session_test.go add context import
- P0-02: wire_gen.go add usageLogRepository parameter
- P0-03: admin_service_stub_test.go add GetGroupAPIKeyCount
- P0-04: admin_basic_handlers_test.go add stubUsageLogRepository

P1 fixes:
- P1-03: group_handler.go GetStats implement real data query

E2E fixes:
- Fix API Key path to /api/v1/keys (user endpoint)

Documentation:
- Update MEMORY.md with latest fixes

backend/internal/handler/admin/admin_basic_handlers_test.go

@@ -17,7 +17,7 @@ func setupAdminRouter() (*gin.Engine, *stubAdminService) {
 	adminSvc := newStubAdminService()
 
 	userHandler := NewUserHandler(adminSvc, nil)
-	groupHandler := NewGroupHandler(adminSvc, nil, nil)
+	groupHandler := NewGroupHandler(adminSvc, nil, nil, &stubUsageLogRepository{})
 	proxyHandler := NewProxyHandler(adminSvc)
 	redeemHandler := NewRedeemHandler(adminSvc, nil)