fix(调度): 修复 outbox 空载写入并稳固回放测试

将 outbox payload 为空时写入 NULL
避免事务因 JSON 解析错误中断
调整回放测试为预置缓存后验证 last_used 更新

测试: go test -tags=integration ./internal/repository

backend/internal/repository/scheduler_snapshot_outbox_integration_test.go

@@ -46,25 +46,12 @@ func TestSchedulerSnapshotOutboxReplay(t *testing.T) {
 		Extra:       map[string]any{},
 	}
 	require.NoError(t, accountRepo.Create(ctx, account))
+	require.NoError(t, cache.SetAccount(ctx, account))
 
 	svc := service.NewSchedulerSnapshotService(cache, outboxRepo, accountRepo, nil, cfg)
 	svc.Start()
 	t.Cleanup(svc.Stop)
 
-	bucket := service.SchedulerBucket{GroupID: 0, Platform: service.PlatformOpenAI, Mode: service.SchedulerModeSingle}
-	require.Eventually(t, func() bool {
-		accounts, hit, err := cache.GetSnapshot(ctx, bucket)
-		if err != nil || !hit {
-			return false
-		}
-		for _, acc := range accounts {
-			if acc.ID == account.ID {
-				return true
-			}
-		}
-		return false
-	}, 5*time.Second, 100*time.Millisecond)
-
 	require.NoError(t, accountRepo.UpdateLastUsed(ctx, account.ID))
 	updated, err := accountRepo.GetByID(ctx, account.ID)
 	require.NoError(t, err)