feat(memory): memory extraction replay operator API

**Context:** ROADMAP.md → Memory security → Memory extraction replay
**Related:** #27

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## Component

API or orchestration

## Describe the feature

Operator API (e.g. `POST /memory/extraction-jobs` or `start_memory_extraction_job`) to **re-run failed PR-review extraction** after webhook or Lambda errors.

## Use case

Transient failures during review-comment extraction leave gaps in learning. Operators should replay without re-running the full agent task.

## Proposed solution

1. Durable job record: PR URL, task_id, last error, status.
2. Idempotent replay from stored webhook payload or GitHub API fetch.
3. CLI: `bgagent memory replay-extraction `.
4. Link outcomes to #27 memory loop metrics.

## Other information

- Design context: `docs/design/MEMORY.md`, `docs/design/ORCHESTRATOR.md`.

- [ ] This might be a breaking change

Beginne mit docs/design/MEMORY.md und docs/design/ORCHESTRATOR.md und verfolge dann die vorhandenen Einstiegspunkte für die Memory-Extraktion und Orchestrierung. Definiere den dauerhaften Job-Datensatz, den idempotenten Replay-Pfad, die Operator-API und die `bgagent memory replay-extraction` CLI, wobei die Ergebnisse mit den Metriken in #27 verknüpft werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.