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/wire.go

@@ -70,6 +70,16 @@ func ProvideSettingHandler(settingService *service.SettingService, buildInfo Bui
 	return NewSettingHandler(settingService, buildInfo.Version)
 }
 
+// ProvideGroupHandler creates GroupHandler with all dependencies
+func ProvideGroupHandler(
+	adminService service.AdminService,
+	dashboardService *service.DashboardService,
+	groupCapacityService *service.GroupCapacityService,
+	usageLogRepo service.UsageLogRepository,
+) *admin.GroupHandler {
+	return admin.NewGroupHandler(adminService, dashboardService, groupCapacityService, usageLogRepo)
+}
+
 // ProvideHandlers creates the Handlers struct
 func ProvideHandlers(
 	authHandler *AuthHandler,
@@ -126,7 +136,7 @@ var ProviderSet = wire.NewSet(
 	// Admin handlers
 	admin.NewDashboardHandler,
 	admin.NewUserHandler,
-	admin.NewGroupHandler,
+	ProvideGroupHandler,
 	admin.NewAccountHandler,
 	admin.NewAnnouncementHandler,
 	admin.NewDataManagementHandler,