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/runtime/runtime.go

@@ -521,11 +521,6 @@ func init() {
 	channelmonitorDescIntervalSeconds := channelmonitorFields[8].Descriptor()
 	// channelmonitor.IntervalSecondsValidator is a validator for the "interval_seconds" field. It is called by the builders before save.
 	channelmonitor.IntervalSecondsValidator = channelmonitorDescIntervalSeconds.Validators[0].(func(int) error)
-	channelmonitordailyrollupMixin := schema.ChannelMonitorDailyRollup{}.Mixin()
-	channelmonitordailyrollupMixinHooks0 := channelmonitordailyrollupMixin[0].Hooks()
-	channelmonitordailyrollup.Hooks[0] = channelmonitordailyrollupMixinHooks0[0]
-	channelmonitordailyrollupMixinInters0 := channelmonitordailyrollupMixin[0].Interceptors()
-	channelmonitordailyrollup.Interceptors[0] = channelmonitordailyrollupMixinInters0[0]
 	channelmonitordailyrollupFields := schema.ChannelMonitorDailyRollup{}.Fields()
 	_ = channelmonitordailyrollupFields
 	// channelmonitordailyrollupDescModel is the schema descriptor for model field.
@@ -592,11 +587,6 @@ func init() {
 	channelmonitordailyrollup.DefaultComputedAt = channelmonitordailyrollupDescComputedAt.Default.(func() time.Time)
 	// channelmonitordailyrollup.UpdateDefaultComputedAt holds the default value on update for the computed_at field.
 	channelmonitordailyrollup.UpdateDefaultComputedAt = channelmonitordailyrollupDescComputedAt.UpdateDefault.(func() time.Time)
-	channelmonitorhistoryMixin := schema.ChannelMonitorHistory{}.Mixin()
-	channelmonitorhistoryMixinHooks0 := channelmonitorhistoryMixin[0].Hooks()
-	channelmonitorhistory.Hooks[0] = channelmonitorhistoryMixinHooks0[0]
-	channelmonitorhistoryMixinInters0 := channelmonitorhistoryMixin[0].Interceptors()
-	channelmonitorhistory.Interceptors[0] = channelmonitorhistoryMixinInters0[0]
 	channelmonitorhistoryFields := schema.ChannelMonitorHistory{}.Fields()
 	_ = channelmonitorhistoryFields
 	// channelmonitorhistoryDescModel is the schema descriptor for model field.