feat(webhooks): add webhook support for external integrations

Add WebhookConfig model, CRUD API endpoints, HMAC-SHA256 signed delivery,
and Celery-based async dispatch with retry/backoff for document events
(document.uploaded, document.processed, document.failed).

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

app/api/__init__.py

@@ -23,6 +23,7 @@ from app.api.url_upload import router as url_upload_router
 
 # Import all the individual routers
 from app.api.user import router as user_router
+from app.api.webhooks import router as webhooks_router
 
 # Set up logging
 logger = logging.getLogger(__name__)
@@ -46,3 +47,4 @@ router.include_router(url_upload_router)
 router.include_router(search_router)
 router.include_router(queue_router)
 router.include_router(saved_searches_router)
+router.include_router(webhooks_router)