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/usersubscription_query.go

@@ -4,6 +4,7 @@ package ent
 
 import (
 	"context"
+	"database/sql/driver"
 	"fmt"
 	"math"
 
@@ -13,6 +14,7 @@ import (
 	"entgo.io/ent/schema/field"
 	"github.com/Wei-Shaw/sub2api/ent/group"
 	"github.com/Wei-Shaw/sub2api/ent/predicate"
+	"github.com/Wei-Shaw/sub2api/ent/usagelog"
 	"github.com/Wei-Shaw/sub2api/ent/user"
 	"github.com/Wei-Shaw/sub2api/ent/usersubscription"
 )
@@ -27,6 +29,7 @@ type UserSubscriptionQuery struct {
 	withUser           *UserQuery
 	withGroup          *GroupQuery
 	withAssignedByUser *UserQuery
+	withUsageLogs      *UsageLogQuery
 	// intermediate query (i.e. traversal path).
 	sql  *sql.Selector
 	path func(context.Context) (*sql.Selector, error)
@@ -129,6 +132,28 @@ func (_q *UserSubscriptionQuery) QueryAssignedByUser() *UserQuery {
 	return query
 }
 
+// QueryUsageLogs chains the current query on the "usage_logs" edge.
+func (_q *UserSubscriptionQuery) QueryUsageLogs() *UsageLogQuery {
+	query := (&UsageLogClient{config: _q.config}).Query()
+	query.path = func(ctx context.Context) (fromU *sql.Selector, err error) {
+		if err := _q.prepareQuery(ctx); err != nil {
+			return nil, err
+		}
+		selector := _q.sqlQuery(ctx)
+		if err := selector.Err(); err != nil {
+			return nil, err
+		}
+		step := sqlgraph.NewStep(
+			sqlgraph.From(usersubscription.Table, usersubscription.FieldID, selector),
+			sqlgraph.To(usagelog.Table, usagelog.FieldID),
+			sqlgraph.Edge(sqlgraph.O2M, false, usersubscription.UsageLogsTable, usersubscription.UsageLogsColumn),
+		)
+		fromU = sqlgraph.SetNeighbors(_q.driver.Dialect(), step)
+		return fromU, nil
+	}
+	return query
+}
+
 // First returns the first UserSubscription entity from the query.
 // Returns a *NotFoundError when no UserSubscription was found.
 func (_q *UserSubscriptionQuery) First(ctx context.Context) (*UserSubscription, error) {
@@ -324,6 +349,7 @@ func (_q *UserSubscriptionQuery) Clone() *UserSubscriptionQuery {
 		withUser:           _q.withUser.Clone(),
 		withGroup:          _q.withGroup.Clone(),
 		withAssignedByUser: _q.withAssignedByUser.Clone(),
+		withUsageLogs:      _q.withUsageLogs.Clone(),
 		// clone intermediate query.
 		sql:  _q.sql.Clone(),
 		path: _q.path,
@@ -363,6 +389,17 @@ func (_q *UserSubscriptionQuery) WithAssignedByUser(opts ...func(*UserQuery)) *U
 	return _q
 }
 
+// WithUsageLogs tells the query-builder to eager-load the nodes that are connected to
+// the "usage_logs" edge. The optional arguments are used to configure the query builder of the edge.
+func (_q *UserSubscriptionQuery) WithUsageLogs(opts ...func(*UsageLogQuery)) *UserSubscriptionQuery {
+	query := (&UsageLogClient{config: _q.config}).Query()
+	for _, opt := range opts {
+		opt(query)
+	}
+	_q.withUsageLogs = query
+	return _q
+}
+
 // GroupBy is used to group vertices by one or more fields/columns.
 // It is often used with aggregate functions, like: count, max, mean, min, sum.
 //
@@ -441,10 +478,11 @@ func (_q *UserSubscriptionQuery) sqlAll(ctx context.Context, hooks ...queryHook)
 	var (
 		nodes       = []*UserSubscription{}
 		_spec       = _q.querySpec()
-		loadedTypes = [3]bool{
+		loadedTypes = [4]bool{
 			_q.withUser != nil,
 			_q.withGroup != nil,
 			_q.withAssignedByUser != nil,
+			_q.withUsageLogs != nil,
 		}
 	)
 	_spec.ScanValues = func(columns []string) ([]any, error) {
@@ -483,6 +521,13 @@ func (_q *UserSubscriptionQuery) sqlAll(ctx context.Context, hooks ...queryHook)
 			return nil, err
 		}
 	}
+	if query := _q.withUsageLogs; query != nil {
+		if err := _q.loadUsageLogs(ctx, query, nodes,
+			func(n *UserSubscription) { n.Edges.UsageLogs = []*UsageLog{} },
+			func(n *UserSubscription, e *UsageLog) { n.Edges.UsageLogs = append(n.Edges.UsageLogs, e) }); err != nil {
+			return nil, err
+		}
+	}
 	return nodes, nil
 }
 
@@ -576,6 +621,39 @@ func (_q *UserSubscriptionQuery) loadAssignedByUser(ctx context.Context, query *
 	}
 	return nil
 }
+func (_q *UserSubscriptionQuery) loadUsageLogs(ctx context.Context, query *UsageLogQuery, nodes []*UserSubscription, init func(*UserSubscription), assign func(*UserSubscription, *UsageLog)) error {
+	fks := make([]driver.Value, 0, len(nodes))
+	nodeids := make(map[int64]*UserSubscription)
+	for i := range nodes {
+		fks = append(fks, nodes[i].ID)
+		nodeids[nodes[i].ID] = nodes[i]
+		if init != nil {
+			init(nodes[i])
+		}
+	}
+	if len(query.ctx.Fields) > 0 {
+		query.ctx.AppendFieldOnce(usagelog.FieldSubscriptionID)
+	}
+	query.Where(predicate.UsageLog(func(s *sql.Selector) {
+		s.Where(sql.InValues(s.C(usersubscription.UsageLogsColumn), fks...))
+	}))
+	neighbors, err := query.All(ctx)
+	if err != nil {
+		return err
+	}
+	for _, n := range neighbors {
+		fk := n.SubscriptionID
+		if fk == nil {
+			return fmt.Errorf(`foreign-key "subscription_id" is nil for node %v`, n.ID)
+		}
+		node, ok := nodeids[*fk]
+		if !ok {
+			return fmt.Errorf(`unexpected referenced foreign-key "subscription_id" returned %v for node %v`, *fk, n.ID)
+		}
+		assign(node, n)
+	}
+	return nil
+}
 
 func (_q *UserSubscriptionQuery) sqlCount(ctx context.Context) (int, error) {
 	_spec := _q.querySpec()