Add the async job submission API, webhook state notifications, and
artifact serving endpoints on top of the Phase 3 queue infrastructure.
Key changes:
- POST /jobs: async job submission with 202 + job_id ULID; optional
state_webhook_url for push notifications on state transitions
- GET /jobs/{id}: job status polling with result, error, and artifact
metadata; artifacts <= 256KB inlined, larger ones by URL reference
- GET /jobs/{id}/artifacts/{name}: raw artifact data serving
- Webhook dispatcher: at-least-once delivery with exponential backoff
(5 retries); optional HMAC-SHA256 signing (X-Foreman-Signature)
- ADR-0014: state_webhook_url only honored on POST /jobs, not sync
/api/chat (caller already blocks for result)
- Comprehensive tests for /jobs lifecycle, webhook delivery, HMAC
verification, artifact inline/URL threshold, and TTL pruning
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
