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/migrate/schema.go

@@ -464,7 +464,6 @@ var (
 	// ChannelMonitorDailyRollupsColumns holds the columns for the "channel_monitor_daily_rollups" table.
 	ChannelMonitorDailyRollupsColumns = []*schema.Column{
 		{Name: "id", Type: field.TypeInt64, Increment: true},
-		{Name: "deleted_at", Type: field.TypeTime, Nullable: true, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "model", Type: field.TypeString, Size: 200},
 		{Name: "bucket_date", Type: field.TypeTime, SchemaType: map[string]string{"postgres": "date"}},
 		{Name: "total_checks", Type: field.TypeInt, Default: 0},
@@ -488,7 +487,7 @@ var (
 		ForeignKeys: []*schema.ForeignKey{
 			{
 				Symbol:     "channel_monitor_daily_rollups_channel_monitors_daily_rollups",
-				Columns:    []*schema.Column{ChannelMonitorDailyRollupsColumns[15]},
+				Columns:    []*schema.Column{ChannelMonitorDailyRollupsColumns[14]},
 				RefColumns: []*schema.Column{ChannelMonitorsColumns[0]},
 				OnDelete:   schema.Cascade,
 			},
@@ -497,19 +496,18 @@ var (
 			{
 				Name:    "channelmonitordailyrollup_monitor_id_model_bucket_date",
 				Unique:  true,
-				Columns: []*schema.Column{ChannelMonitorDailyRollupsColumns[15], ChannelMonitorDailyRollupsColumns[2], ChannelMonitorDailyRollupsColumns[3]},
+				Columns: []*schema.Column{ChannelMonitorDailyRollupsColumns[14], ChannelMonitorDailyRollupsColumns[1], ChannelMonitorDailyRollupsColumns[2]},
 			},
 			{
 				Name:    "channelmonitordailyrollup_bucket_date",
 				Unique:  false,
-				Columns: []*schema.Column{ChannelMonitorDailyRollupsColumns[3]},
+				Columns: []*schema.Column{ChannelMonitorDailyRollupsColumns[2]},
 			},
 		},
 	}
 	// ChannelMonitorHistoriesColumns holds the columns for the "channel_monitor_histories" table.
 	ChannelMonitorHistoriesColumns = []*schema.Column{
 		{Name: "id", Type: field.TypeInt64, Increment: true},
-		{Name: "deleted_at", Type: field.TypeTime, Nullable: true, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "model", Type: field.TypeString, Size: 200},
 		{Name: "status", Type: field.TypeEnum, Enums: []string{"operational", "degraded", "failed", "error"}},
 		{Name: "latency_ms", Type: field.TypeInt, Nullable: true},
@@ -526,7 +524,7 @@ var (
 		ForeignKeys: []*schema.ForeignKey{
 			{
 				Symbol:     "channel_monitor_histories_channel_monitors_history",
-				Columns:    []*schema.Column{ChannelMonitorHistoriesColumns[8]},
+				Columns:    []*schema.Column{ChannelMonitorHistoriesColumns[7]},
 				RefColumns: []*schema.Column{ChannelMonitorsColumns[0]},
 				OnDelete:   schema.Cascade,
 			},
@@ -535,12 +533,12 @@ var (
 			{
 				Name:    "channelmonitorhistory_monitor_id_model_checked_at",
 				Unique:  false,
-				Columns: []*schema.Column{ChannelMonitorHistoriesColumns[8], ChannelMonitorHistoriesColumns[2], ChannelMonitorHistoriesColumns[7]},
+				Columns: []*schema.Column{ChannelMonitorHistoriesColumns[7], ChannelMonitorHistoriesColumns[1], ChannelMonitorHistoriesColumns[6]},
 			},
 			{
 				Name:    "channelmonitorhistory_checked_at",
 				Unique:  false,
-				Columns: []*schema.Column{ChannelMonitorHistoriesColumns[7]},
+				Columns: []*schema.Column{ChannelMonitorHistoriesColumns[6]},
 			},
 		},
 	}