The per-user notification functions (notify_user_document_processed /
notify_user_document_failed) were defined but never called from the
document processing pipeline.
- Call notify_user_document_processed in finalize_document_storage
when owner_id is available (creates in-app + email/webhook notifications)
- Add _dispatch_user_failure_notification helper to celery_app.py that
extracts file_id from failed task args and dispatches
notify_user_document_failed for document pipeline tasks
- Add comprehensive tests for both success and failure notification paths
Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>