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/internal/repository/migrations_schema_integration_test.go

@@ -53,6 +53,20 @@ func TestMigrationsRunner_IsIdempotent_AndSchemaIsUpToDate(t *testing.T) {
 	var uagRegclass sql.NullString
 	require.NoError(t, tx.QueryRowContext(context.Background(), "SELECT to_regclass('public.user_allowed_groups')").Scan(&uagRegclass))
 	require.True(t, uagRegclass.Valid, "expected user_allowed_groups table to exist")
+
+	// user_subscriptions: deleted_at for soft delete support (migration 012)
+	requireColumn(t, tx, "user_subscriptions", "deleted_at", "timestamp with time zone", 0, true)
+
+	// orphan_allowed_groups_audit table should exist (migration 013)
+	var orphanAuditRegclass sql.NullString
+	require.NoError(t, tx.QueryRowContext(context.Background(), "SELECT to_regclass('public.orphan_allowed_groups_audit')").Scan(&orphanAuditRegclass))
+	require.True(t, orphanAuditRegclass.Valid, "expected orphan_allowed_groups_audit table to exist")
+
+	// account_groups: created_at should be timestamptz
+	requireColumn(t, tx, "account_groups", "created_at", "timestamp with time zone", 0, false)
+
+	// user_allowed_groups: created_at should be timestamptz
+	requireColumn(t, tx, "user_allowed_groups", "created_at", "timestamp with time zone", 0, false)
 }
 
 func requireColumn(t *testing.T, tx *sql.Tx, table, column, dataType string, maxLen int, nullable bool) {