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/intercept/intercept.go

@@ -16,6 +16,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/ent/proxy"
 	"github.com/Wei-Shaw/sub2api/ent/redeemcode"
 	"github.com/Wei-Shaw/sub2api/ent/setting"
+	"github.com/Wei-Shaw/sub2api/ent/usagelog"
 	"github.com/Wei-Shaw/sub2api/ent/user"
 	"github.com/Wei-Shaw/sub2api/ent/userallowedgroup"
 	"github.com/Wei-Shaw/sub2api/ent/usersubscription"
@@ -266,6 +267,33 @@ func (f TraverseSetting) Traverse(ctx context.Context, q ent.Query) error {
 	return fmt.Errorf("unexpected query type %T. expect *ent.SettingQuery", q)
 }
 
+// The UsageLogFunc type is an adapter to allow the use of ordinary function as a Querier.
+type UsageLogFunc func(context.Context, *ent.UsageLogQuery) (ent.Value, error)
+
+// Query calls f(ctx, q).
+func (f UsageLogFunc) Query(ctx context.Context, q ent.Query) (ent.Value, error) {
+	if q, ok := q.(*ent.UsageLogQuery); ok {
+		return f(ctx, q)
+	}
+	return nil, fmt.Errorf("unexpected query type %T. expect *ent.UsageLogQuery", q)
+}
+
+// The TraverseUsageLog type is an adapter to allow the use of ordinary function as Traverser.
+type TraverseUsageLog func(context.Context, *ent.UsageLogQuery) error
+
+// Intercept is a dummy implementation of Intercept that returns the next Querier in the pipeline.
+func (f TraverseUsageLog) Intercept(next ent.Querier) ent.Querier {
+	return next
+}
+
+// Traverse calls f(ctx, q).
+func (f TraverseUsageLog) Traverse(ctx context.Context, q ent.Query) error {
+	if q, ok := q.(*ent.UsageLogQuery); ok {
+		return f(ctx, q)
+	}
+	return fmt.Errorf("unexpected query type %T. expect *ent.UsageLogQuery", q)
+}
+
 // The UserFunc type is an adapter to allow the use of ordinary function as a Querier.
 type UserFunc func(context.Context, *ent.UserQuery) (ent.Value, error)
 
@@ -364,6 +392,8 @@ func NewQuery(q ent.Query) (Query, error) {
 		return &query[*ent.RedeemCodeQuery, predicate.RedeemCode, redeemcode.OrderOption]{typ: ent.TypeRedeemCode, tq: q}, nil
 	case *ent.SettingQuery:
 		return &query[*ent.SettingQuery, predicate.Setting, setting.OrderOption]{typ: ent.TypeSetting, tq: q}, nil
+	case *ent.UsageLogQuery:
+		return &query[*ent.UsageLogQuery, predicate.UsageLog, usagelog.OrderOption]{typ: ent.TypeUsageLog, tq: q}, nil
 	case *ent.UserQuery:
 		return &query[*ent.UserQuery, predicate.User, user.OrderOption]{typ: ent.TypeUser, tq: q}, nil
 	case *ent.UserAllowedGroupQuery: