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/ent.go

@@ -19,6 +19,7 @@ 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"
@@ -89,6 +90,7 @@ func checkColumn(t, c string) error {
 			proxy.Table:            proxy.ValidColumn,
 			redeemcode.Table:       redeemcode.ValidColumn,
 			setting.Table:          setting.ValidColumn,
+			usagelog.Table:         usagelog.ValidColumn,
 			user.Table:             user.ValidColumn,
 			userallowedgroup.Table: userallowedgroup.ValidColumn,
 			usersubscription.Table: usersubscription.ValidColumn,