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_update.go

@@ -29,26 +29,6 @@ func (_u *ChannelMonitorHistoryUpdate) Where(ps ...predicate.ChannelMonitorHisto
 	return _u
 }
 
-// SetDeletedAt sets the "deleted_at" field.
-func (_u *ChannelMonitorHistoryUpdate) SetDeletedAt(v time.Time) *ChannelMonitorHistoryUpdate {
-	_u.mutation.SetDeletedAt(v)
-	return _u
-}
-
-// SetNillableDeletedAt sets the "deleted_at" field if the given value is not nil.
-func (_u *ChannelMonitorHistoryUpdate) SetNillableDeletedAt(v *time.Time) *ChannelMonitorHistoryUpdate {
-	if v != nil {
-		_u.SetDeletedAt(*v)
-	}
-	return _u
-}
-
-// ClearDeletedAt clears the value of the "deleted_at" field.
-func (_u *ChannelMonitorHistoryUpdate) ClearDeletedAt() *ChannelMonitorHistoryUpdate {
-	_u.mutation.ClearDeletedAt()
-	return _u
-}
-
 // SetMonitorID sets the "monitor_id" field.
 func (_u *ChannelMonitorHistoryUpdate) SetMonitorID(v int64) *ChannelMonitorHistoryUpdate {
 	_u.mutation.SetMonitorID(v)
@@ -257,12 +237,6 @@ func (_u *ChannelMonitorHistoryUpdate) sqlSave(ctx context.Context) (_node int,
 			}
 		}
 	}
-	if value, ok := _u.mutation.DeletedAt(); ok {
-		_spec.SetField(channelmonitorhistory.FieldDeletedAt, field.TypeTime, value)
-	}
-	if _u.mutation.DeletedAtCleared() {
-		_spec.ClearField(channelmonitorhistory.FieldDeletedAt, field.TypeTime)
-	}
 	if value, ok := _u.mutation.Model(); ok {
 		_spec.SetField(channelmonitorhistory.FieldModel, field.TypeString, value)
 	}
@@ -345,26 +319,6 @@ type ChannelMonitorHistoryUpdateOne struct {
 	mutation *ChannelMonitorHistoryMutation
 }
 
-// SetDeletedAt sets the "deleted_at" field.
-func (_u *ChannelMonitorHistoryUpdateOne) SetDeletedAt(v time.Time) *ChannelMonitorHistoryUpdateOne {
-	_u.mutation.SetDeletedAt(v)
-	return _u
-}
-
-// SetNillableDeletedAt sets the "deleted_at" field if the given value is not nil.
-func (_u *ChannelMonitorHistoryUpdateOne) SetNillableDeletedAt(v *time.Time) *ChannelMonitorHistoryUpdateOne {
-	if v != nil {
-		_u.SetDeletedAt(*v)
-	}
-	return _u
-}
-
-// ClearDeletedAt clears the value of the "deleted_at" field.
-func (_u *ChannelMonitorHistoryUpdateOne) ClearDeletedAt() *ChannelMonitorHistoryUpdateOne {
-	_u.mutation.ClearDeletedAt()
-	return _u
-}
-
 // SetMonitorID sets the "monitor_id" field.
 func (_u *ChannelMonitorHistoryUpdateOne) SetMonitorID(v int64) *ChannelMonitorHistoryUpdateOne {
 	_u.mutation.SetMonitorID(v)
@@ -603,12 +557,6 @@ func (_u *ChannelMonitorHistoryUpdateOne) sqlSave(ctx context.Context) (_node *C
 			}
 		}
 	}
-	if value, ok := _u.mutation.DeletedAt(); ok {
-		_spec.SetField(channelmonitorhistory.FieldDeletedAt, field.TypeTime, value)
-	}
-	if _u.mutation.DeletedAtCleared() {
-		_spec.ClearField(channelmonitorhistory.FieldDeletedAt, field.TypeTime)
-	}
 	if value, ok := _u.mutation.Model(); ok {
 		_spec.SetField(channelmonitorhistory.FieldModel, field.TypeString, value)
 	}