feat(orchestration): admission backlog observability

**Context:** ROADMAP.md → Admission backlog observability
**Related:** admission-queue-deferred-pickup draft, #331 (closed)

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## Component

API or orchestration

## Describe the feature

**CloudWatch metric and alarm** when `SUBMITTED` task depth exceeds an operator threshold—surfacing capacity and admission health before users see failures.

## Use case

Operators cannot distinguish healthy queueing from stuck hydration or admission starvation. Proactive metrics reduce incident time.

## Proposed solution

1. Scheduled or stream-derived gauge: count tasks in `SUBMITTED` per user/fleet.
2. Dashboard widget + alarm with configurable threshold.
3. Optional dimension: time-in-SUBMITTED p95.
4. Document runbook: scale concurrency, investigate hydrator, enable admission queue when shipped.

## Other information

- Pair with **admission queue** draft.
- Design context: `docs/design/OBSERVABILITY.md`, `docs/design/ORCHESTRATOR.md`.

- [ ] This might be a breaking change

ROADMAP.md と docs/design/OBSERVABILITY.md および docs/design/ORCHESTRATOR.md の設計コンテキストから始め、次に admission-queue-deferred-pickup のドラフトと issue #331 を確認してください。SUBMITTED-depth と任意の time-in-SUBMITTED メトリクス、ダッシュボードと設定可能なアラーム、およびオペレーター向け runbook の実装範囲を定義してください。完了時には、合意された observability とドキュメントの項目をカバーする必要があります。

索引モデルが issue の本文から書いたものです。