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/account/where.go

@@ -495,26 +495,6 @@ func ProxyIDNotIn(vs ...int64) predicate.Account {
 	return predicate.Account(sql.FieldNotIn(FieldProxyID, vs...))
 }
 
-// ProxyIDGT applies the GT predicate on the "proxy_id" field.
-func ProxyIDGT(v int64) predicate.Account {
-	return predicate.Account(sql.FieldGT(FieldProxyID, v))
-}
-
-// ProxyIDGTE applies the GTE predicate on the "proxy_id" field.
-func ProxyIDGTE(v int64) predicate.Account {
-	return predicate.Account(sql.FieldGTE(FieldProxyID, v))
-}
-
-// ProxyIDLT applies the LT predicate on the "proxy_id" field.
-func ProxyIDLT(v int64) predicate.Account {
-	return predicate.Account(sql.FieldLT(FieldProxyID, v))
-}
-
-// ProxyIDLTE applies the LTE predicate on the "proxy_id" field.
-func ProxyIDLTE(v int64) predicate.Account {
-	return predicate.Account(sql.FieldLTE(FieldProxyID, v))
-}
-
 // ProxyIDIsNil applies the IsNil predicate on the "proxy_id" field.
 func ProxyIDIsNil() predicate.Account {
 	return predicate.Account(sql.FieldIsNull(FieldProxyID))
@@ -1153,6 +1133,52 @@ func HasGroupsWith(preds ...predicate.Group) predicate.Account {
 	})
 }
 
+// HasProxy applies the HasEdge predicate on the "proxy" edge.
+func HasProxy() predicate.Account {
+	return predicate.Account(func(s *sql.Selector) {
+		step := sqlgraph.NewStep(
+			sqlgraph.From(Table, FieldID),
+			sqlgraph.Edge(sqlgraph.M2O, false, ProxyTable, ProxyColumn),
+		)
+		sqlgraph.HasNeighbors(s, step)
+	})
+}
+
+// HasProxyWith applies the HasEdge predicate on the "proxy" edge with a given conditions (other predicates).
+func HasProxyWith(preds ...predicate.Proxy) predicate.Account {
+	return predicate.Account(func(s *sql.Selector) {
+		step := newProxyStep()
+		sqlgraph.HasNeighborsWith(s, step, func(s *sql.Selector) {
+			for _, p := range preds {
+				p(s)
+			}
+		})
+	})
+}
+
+// HasUsageLogs applies the HasEdge predicate on the "usage_logs" edge.
+func HasUsageLogs() predicate.Account {
+	return predicate.Account(func(s *sql.Selector) {
+		step := sqlgraph.NewStep(
+			sqlgraph.From(Table, FieldID),
+			sqlgraph.Edge(sqlgraph.O2M, false, UsageLogsTable, UsageLogsColumn),
+		)
+		sqlgraph.HasNeighbors(s, step)
+	})
+}
+
+// HasUsageLogsWith applies the HasEdge predicate on the "usage_logs" edge with a given conditions (other predicates).
+func HasUsageLogsWith(preds ...predicate.UsageLog) predicate.Account {
+	return predicate.Account(func(s *sql.Selector) {
+		step := newUsageLogsStep()
+		sqlgraph.HasNeighborsWith(s, step, func(s *sql.Selector) {
+			for _, p := range preds {
+				p(s)
+			}
+		})
+	})
+}
+
 // HasAccountGroups applies the HasEdge predicate on the "account_groups" edge.
 func HasAccountGroups() predicate.Account {
 	return predicate.Account(func(s *sql.Selector) {