feat(automation): add Zapier and Make.com integration

Add REST hooks subscription endpoints, incoming action endpoints, and
Zapier-compatible flat payload format for automation platform integration.

- AutomationHook model for webhook subscriptions
- POST /api/automation/hooks/subscribe and DELETE /hooks/{id}
- GET /api/automation/triggers/sample/{event} for Zapier field mapping
- POST /api/automation/actions/upload for incoming document uploads
- Celery task with retry for async hook delivery
- Integration with existing webhook dispatch flow
- 30 passing tests covering all new functionality

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/conftest.py

@@ -61,6 +61,7 @@ from app.main import app as fastapi_app  # noqa: E402
 # Import models to register them with SQLAlchemy Base
 from app.models import (  # noqa: F401, E402
     ApiToken,
+    AutomationHook,
     DocumentMetadata,
     FileRecord,
     Pipeline,