fix(channel-monitor): drop soft delete, refactor feature flag to declarative form

### 后端修复：日志表不该用软删除

channel_monitor_histories / channel_monitor_daily_rollups 都是日志/聚合表，
没有恢复需求。110 里加的 SoftDeleteMixin 会让 DELETE 自动变成 UPDATE deleted_at，
导致行和索引只增不减，徒增磁盘占用和查询成本。

改回分批物理删（参考 OpsCleanupService.deleteOldRowsByID 模板）：

- ent schema 移除 SoftDeleteMixin，重新 go generate
- repo 新增 deleteChannelMonitorBatched 辅助 + 两条 prune SQL 常量
  （WITH batch AS SELECT id LIMIT 5000 → DELETE IN batch）
- DeleteHistoryBefore / DeleteRollupsBefore 改调分批 raw SQL
- 移除 ComputeAvailability / ComputeAvailabilityForMonitors / UpsertDailyRollupsFor /
  ListLatestPerModel / ListLatestForMonitorIDs / ListRecentHistoryForMonitors 等
  raw SQL 中的 deleted_at IS NULL 过滤
- UpsertDailyRollupsFor 的 ON CONFLICT 去掉 deleted_at = NULL 重置
- migration 111 DROP COLUMN deleted_at + 对应索引（110 已部署但 maintenance
  首跑在次日 02:00，此时尚无业务数据在依赖软删除）

### 前端重构：feature flag 声明式 + 复用

AppSidebar.vue 里 7 处 `...(flag ? [item] : [])` 样板代码删光，改为 NavItem 加
featureFlag?: () => boolean | undefined 字段，加一个 applyFeatureFlags 递归
过滤（含 children）。语义统一为 `!== false`（宽容策略，undefined 时默认显示，
避免 public settings 未加载完成时菜单闪烁消失 — 对应用户反馈"刷新后菜单消失
要去保存设置才回来"）。

- 集中声明 4 个 flag getter：flagChannelMonitor / flagPayment /
  flagOpsMonitoring / flagAdminPayment
- 提取 buildSelfNavItems 复用用户端主菜单和管理员"我的账户"子菜单
- 未来新增开关：在统一位置加一个 flag getter + 给对应 NavItem 加字段
  （不用再动渲染逻辑）

bump 0.1.114.29

backend/ent/channelmonitorhistory.go

@@ -18,8 +18,6 @@ type ChannelMonitorHistory struct {
 	config `json:"-"`
 	// ID of the ent.
 	ID int64 `json:"id,omitempty"`
-	// DeletedAt holds the value of the "deleted_at" field.
-	DeletedAt *time.Time `json:"deleted_at,omitempty"`
 	// MonitorID holds the value of the "monitor_id" field.
 	MonitorID int64 `json:"monitor_id,omitempty"`
 	// Model holds the value of the "model" field.
@@ -69,7 +67,7 @@ func (*ChannelMonitorHistory) scanValues(columns []string) ([]any, error) {
 			values[i] = new(sql.NullInt64)
 		case channelmonitorhistory.FieldModel, channelmonitorhistory.FieldStatus, channelmonitorhistory.FieldMessage:
 			values[i] = new(sql.NullString)
-		case channelmonitorhistory.FieldDeletedAt, channelmonitorhistory.FieldCheckedAt:
+		case channelmonitorhistory.FieldCheckedAt:
 			values[i] = new(sql.NullTime)
 		default:
 			values[i] = new(sql.UnknownType)
@@ -92,13 +90,6 @@ func (_m *ChannelMonitorHistory) assignValues(columns []string, values []any) er
 				return fmt.Errorf("unexpected type %T for field id", value)
 			}
 			_m.ID = int64(value.Int64)
-		case channelmonitorhistory.FieldDeletedAt:
-			if value, ok := values[i].(*sql.NullTime); !ok {
-				return fmt.Errorf("unexpected type %T for field deleted_at", values[i])
-			} else if value.Valid {
-				_m.DeletedAt = new(time.Time)
-				*_m.DeletedAt = value.Time
-			}
 		case channelmonitorhistory.FieldMonitorID:
 			if value, ok := values[i].(*sql.NullInt64); !ok {
 				return fmt.Errorf("unexpected type %T for field monitor_id", values[i])
@@ -184,11 +175,6 @@ func (_m *ChannelMonitorHistory) String() string {
 	var builder strings.Builder
 	builder.WriteString("ChannelMonitorHistory(")
 	builder.WriteString(fmt.Sprintf("id=%v, ", _m.ID))
-	if v := _m.DeletedAt; v != nil {
-		builder.WriteString("deleted_at=")
-		builder.WriteString(v.Format(time.ANSIC))
-	}
-	builder.WriteString(", ")
 	builder.WriteString("monitor_id=")
 	builder.WriteString(fmt.Sprintf("%v", _m.MonitorID))
 	builder.WriteString(", ")