fix(tasks): dispatch per-user notifications on document processed/failed events

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>

app/tasks/finalize_document_storage.py

@@ -21,8 +21,9 @@ from app.tasks.send_to_all import (
 # Import database and logging utils from main
 from app.utils import log_task_progress
 
-# Import notification utility
+# Import notification utilities
 from app.utils.notification import notify_file_processed
+from app.utils.user_notification import notify_user_document_processed
 
 logger = logging.getLogger(__name__)
 
@@ -139,4 +140,15 @@ def finalize_document_storage(self, original_file: str, processed_file: str, met
     except Exception as e:
         logger.warning(f"[WARNING] Failed to send file processed notification: {e}")
 
+    # 6. Send per-user notification
+    if owner_id:
+        try:
+            notify_user_document_processed(
+                owner_id=owner_id,
+                filename=os.path.basename(processed_file),
+                file_id=file_id,
+            )
+        except Exception as e:
+            logger.warning(f"[WARNING] Failed to send per-user processed notification: {e}")
+
     return {"status": "Completed", "file": processed_file}