Full webhook system: CRUD endpoints (list/filter/get/create/update/delete),
WebhookDelivery model for delivery audit trail, dispatch engine with 3-attempt
retry and exponential backoff, Celery task integration with sync fallback,
and webhook firing hooks in runner.py and sweep.py event paths.
Adds backend/engine/runner.py with run_single() that iterates pipeline stages,
renders Jinja2 prompt templates with stage history context, checks/stores response
cache, calls LLM adapters, runs configured scorers, creates StageResult and Score
records, and publishes progress events via Redis pub/sub or in-process EventBus.
Includes 21 passing tests covering all execution paths.
