fix(数据库): 修复默认分组缺失与迁移锁阻塞

通过迁移补种默认 groups 记录，避免新装空分组
迁移锁改为 try lock + 重试并加入超时
写入 usage_logs 时保留 rate_multiplier=0 语义
测试: go test ./...

backend/internal/repository/usage_log_repo.go

@@ -70,9 +70,6 @@ func (r *usageLogRepository) Create(ctx context.Context, log *service.UsageLog)
 	}
 
 	rateMultiplier := log.RateMultiplier
-	if rateMultiplier == 0 {
-		rateMultiplier = 1
-	}
 
 	query := `
 		INSERT INTO usage_logs (