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/proxy/proxy.go

@@ -7,6 +7,7 @@ import (
 
 	"entgo.io/ent"
 	"entgo.io/ent/dialect/sql"
+	"entgo.io/ent/dialect/sql/sqlgraph"
 )
 
 const (
@@ -34,8 +35,17 @@ const (
 	FieldPassword = "password"
 	// FieldStatus holds the string denoting the status field in the database.
 	FieldStatus = "status"
+	// EdgeAccounts holds the string denoting the accounts edge name in mutations.
+	EdgeAccounts = "accounts"
 	// Table holds the table name of the proxy in the database.
 	Table = "proxies"
+	// AccountsTable is the table that holds the accounts relation/edge.
+	AccountsTable = "accounts"
+	// AccountsInverseTable is the table name for the Account entity.
+	// It exists in this package in order to avoid circular dependency with the "account" package.
+	AccountsInverseTable = "accounts"
+	// AccountsColumn is the table column denoting the accounts relation/edge.
+	AccountsColumn = "proxy_id"
 )
 
 // Columns holds all SQL columns for proxy fields.
@@ -150,3 +160,24 @@ func ByPassword(opts ...sql.OrderTermOption) OrderOption {
 func ByStatus(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldStatus, opts...).ToFunc()
 }
+
+// ByAccountsCount orders the results by accounts count.
+func ByAccountsCount(opts ...sql.OrderTermOption) OrderOption {
+	return func(s *sql.Selector) {
+		sqlgraph.OrderByNeighborsCount(s, newAccountsStep(), opts...)
+	}
+}
+
+// ByAccounts orders the results by accounts terms.
+func ByAccounts(term sql.OrderTerm, terms ...sql.OrderTerm) OrderOption {
+	return func(s *sql.Selector) {
+		sqlgraph.OrderByNeighborTerms(s, newAccountsStep(), append([]sql.OrderTerm{term}, terms...)...)
+	}
+}
+func newAccountsStep() *sqlgraph.Step {
+	return sqlgraph.NewStep(
+		sqlgraph.From(Table, FieldID),
+		sqlgraph.To(AccountsInverseTable, FieldID),
+		sqlgraph.Edge(sqlgraph.O2M, true, AccountsTable, AccountsColumn),
+	)
+}