fix(数据层): 修复数据完整性与仓储一致性问题

## 数据完整性修复 (fix-critical-data-integrity)
- 添加 error_translate.go 统一错误转换层
- 修复 nil 输入和 NotFound 错误处理
- 增强仓储层错误一致性

## 仓储一致性修复 (fix-high-repository-consistency)
- Group schema 添加 default_validity_days 字段
- Account schema 添加 proxy edge 关联
- 新增 UsageLog ent schema 定义
- 修复 UpdateBalance/UpdateConcurrency 受影响行数校验

## 数据卫生修复 (fix-medium-data-hygiene)
- UserSubscription 添加软删除支持 (SoftDeleteMixin)
- RedeemCode/Setting 添加硬删除策略文档
- account_groups/user_allowed_groups 的 created_at 声明 timestamptz
- 停止写入 legacy users.allowed_groups 列
- 新增迁移: 011-014 (索引优化、软删除、孤立数据审计、列清理)

## 测试补充
- 添加 UserSubscription 软删除测试
- 添加迁移回归测试
- 添加 NotFound 错误测试

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/ent/tx.go

@@ -28,6 +28,8 @@ type Tx struct {
 	RedeemCode *RedeemCodeClient
 	// Setting is the client for interacting with the Setting builders.
 	Setting *SettingClient
+	// UsageLog is the client for interacting with the UsageLog builders.
+	UsageLog *UsageLogClient
 	// User is the client for interacting with the User builders.
 	User *UserClient
 	// UserAllowedGroup is the client for interacting with the UserAllowedGroup builders.
@@ -172,6 +174,7 @@ func (tx *Tx) init() {
 	tx.Proxy = NewProxyClient(tx.config)
 	tx.RedeemCode = NewRedeemCodeClient(tx.config)
 	tx.Setting = NewSettingClient(tx.config)
+	tx.UsageLog = NewUsageLogClient(tx.config)
 	tx.User = NewUserClient(tx.config)
 	tx.UserAllowedGroup = NewUserAllowedGroupClient(tx.config)
 	tx.UserSubscription = NewUserSubscriptionClient(tx.config)
@@ -238,7 +241,6 @@ func (tx *txDriver) Query(ctx context.Context, query string, args, v any) error
 
 var _ dialect.Driver = (*txDriver)(nil)
 
-// ExecContext 透传到底层事务，用于在 ent 事务中执行原生 SQL（与 ent 写入保持同一事务）。
 // ExecContext allows calling the underlying ExecContext method of the transaction if it is supported by it.
 // See, database/sql#Tx.ExecContext for more information.
 func (tx *txDriver) ExecContext(ctx context.Context, query string, args ...any) (stdsql.Result, error) {
@@ -251,7 +253,6 @@ func (tx *txDriver) ExecContext(ctx context.Context, query string, args ...any)
 	return ex.ExecContext(ctx, query, args...)
 }
 
-// QueryContext 透传到底层事务，用于在 ent 事务中执行原生查询并共享锁语义。
 // QueryContext allows calling the underlying QueryContext method of the transaction if it is supported by it.
 // See, database/sql#Tx.QueryContext for more information.
 func (tx *txDriver) QueryContext(ctx context.Context, query string, args ...any) (*stdsql.Rows, error) {