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_create.go

@@ -11,6 +11,7 @@ import (
 	"entgo.io/ent/dialect/sql"
 	"entgo.io/ent/dialect/sql/sqlgraph"
 	"entgo.io/ent/schema/field"
+	"github.com/Wei-Shaw/sub2api/ent/account"
 	"github.com/Wei-Shaw/sub2api/ent/proxy"
 )
 
@@ -130,6 +131,21 @@ func (_c *ProxyCreate) SetNillableStatus(v *string) *ProxyCreate {
 	return _c
 }
 
+// AddAccountIDs adds the "accounts" edge to the Account entity by IDs.
+func (_c *ProxyCreate) AddAccountIDs(ids ...int64) *ProxyCreate {
+	_c.mutation.AddAccountIDs(ids...)
+	return _c
+}
+
+// AddAccounts adds the "accounts" edges to the Account entity.
+func (_c *ProxyCreate) AddAccounts(v ...*Account) *ProxyCreate {
+	ids := make([]int64, len(v))
+	for i := range v {
+		ids[i] = v[i].ID
+	}
+	return _c.AddAccountIDs(ids...)
+}
+
 // Mutation returns the ProxyMutation object of the builder.
 func (_c *ProxyCreate) Mutation() *ProxyMutation {
 	return _c.mutation
@@ -308,6 +324,22 @@ func (_c *ProxyCreate) createSpec() (*Proxy, *sqlgraph.CreateSpec) {
 		_spec.SetField(proxy.FieldStatus, field.TypeString, value)
 		_node.Status = value
 	}
+	if nodes := _c.mutation.AccountsIDs(); len(nodes) > 0 {
+		edge := &sqlgraph.EdgeSpec{
+			Rel:     sqlgraph.O2M,
+			Inverse: true,
+			Table:   proxy.AccountsTable,
+			Columns: []string{proxy.AccountsColumn},
+			Bidi:    false,
+			Target: &sqlgraph.EdgeTarget{
+				IDSpec: sqlgraph.NewFieldSpec(account.FieldID, field.TypeInt64),
+			},
+		}
+		for _, k := range nodes {
+			edge.Target.Nodes = append(edge.Target.Nodes, k)
+		}
+		_spec.Edges = append(_spec.Edges, edge)
+	}
 	return _node, _spec
 }