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

Beginnen Sie mit ROADMAP.md und dem Designkontext in docs/design/OBSERVABILITY.md und docs/design/ORCHESTRATOR.md, und prüfen Sie anschließend den Entwurf admission-queue-deferred-pickup sowie Issue #331. Definieren Sie den Implementierungsumfang für SUBMITTED-depth und optionale time-in-SUBMITTED-Metriken, das Dashboard und den konfigurierbaren Alarm sowie das Operator-Runbook; der Abschluss sollte die vereinbarten Observability- und Dokumentationsteile abdecken.

Vom Indexierungsmodell aus dem Issue-Text verfasst.