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/celery_app.py

@@ -1,10 +1,14 @@
 # app/celery_app.py
 
+import logging
+
 from celery import Celery
 from celery.signals import task_failure, worker_ready
 
 from app.config import settings
 
+logger = logging.getLogger(__name__)
+
 celery = Celery(
     "document_processor",
     broker=settings.redis_url,
@@ -21,6 +25,62 @@ celery.conf.task_routes = {
     "app.tasks.*": {"queue": "document_processor"},
 }
 
+# Mapping of document pipeline task names to the positional index of ``file_id``
+# in their ``args`` tuple.  Tasks that always pass ``file_id`` as a keyword
+# argument (e.g. ``process_document``, ``finalize_document_storage``) are not
+# listed here — their ``file_id`` is found via ``kwargs`` instead.
+_FILE_ID_ARG_INDEX: dict[str, int] = {
+    "app.tasks.process_with_ocr.process_with_ocr": 1,
+    "app.tasks.extract_metadata_with_gpt.extract_metadata_with_gpt": 2,
+    "app.tasks.embed_metadata_into_pdf.embed_metadata_into_pdf": 3,
+}
+
+
+def _dispatch_user_failure_notification(sender, exception, args: list | None, kwargs: dict | None) -> None:
+    """Best-effort per-user failure notification for document pipeline tasks.
+
+    Extracts ``file_id`` from the failed task's arguments, looks up the owning
+    user from the database, and dispatches a ``document.failed`` notification.
+    """
+    from app.database import SessionLocal
+    from app.models import FileRecord
+    from app.utils.user_notification import notify_user_document_failed
+
+    task_name = sender.name if sender else ""
+    if not task_name.startswith("app.tasks."):
+        return
+
+    # 1. Resolve file_id from kwargs or positional args
+    file_id = (kwargs or {}).get("file_id")
+    if file_id is None:
+        idx = _FILE_ID_ARG_INDEX.get(task_name)
+        if idx is not None and args and len(args) > idx:
+            val = args[idx]
+            if isinstance(val, int):
+                file_id = val
+
+    if file_id is None:
+        return
+
+    # 2. Look up owner from the database
+    with SessionLocal() as db:
+        record = db.query(FileRecord).filter(FileRecord.id == file_id).first()
+        if not record or not record.owner_id:
+            return
+        owner_id = record.owner_id
+        filename = record.original_filename or record.local_filename or "unknown"
+
+    # 3. Dispatch per-user notification
+    import os
+
+    error_msg = f"{type(exception).__name__}: {exception}" if exception else "Unknown error"
+    notify_user_document_failed(
+        owner_id=owner_id,
+        filename=os.path.basename(filename),
+        error=error_msg,
+        file_id=file_id,
+    )
+
 
 @worker_ready.connect
 def init_sentry_on_worker_ready(**kwargs):
@@ -48,6 +108,10 @@ def task_failure_handler(
                 kwargs=kwargs or {},
             )
         except Exception as e:
-            import logging
+            logger.exception(f"Failed to send task failure notification: {e}")
 
-            logging.exception(f"Failed to send task failure notification: {e}")
+    # Also dispatch a per-user failure notification for document pipeline tasks
+    try:
+        _dispatch_user_failure_notification(sender, exception, args, kwargs)
+    except Exception:
+        logger.debug("Could not dispatch per-user failure notification", exc_info=True)