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>

app/api/__init__.py

@@ -8,6 +8,7 @@ from fastapi import APIRouter
 
 from app.api.admin_users import router as admin_users_router
 from app.api.api_tokens import router as api_tokens_router
+from app.api.automation import router as automation_router
 from app.api.azure import router as azure_router
 from app.api.backup import router as backup_router
 from app.api.billing import router as billing_router
@@ -82,3 +83,4 @@ router.include_router(imap_accounts_router)
 router.include_router(integrations_router)
 router.include_router(notifications_router)
 router.include_router(scheduled_jobs_router)
+router.include_router(automation_router)