**Refactor directory scanning and add scan task support**

- Added a new `scan_directory` task to enable folder scans as asynchronous tasks while maintaining compatibility with the immediate backend.
- Introduced the `reconcile` function, replacing `generate_directory` for improved separation of concerns between database writes and read operations.
- Established a unique constraint on `Directory` (`one_directory_name_per_parent`) to prevent duplicate directories under the same parent.
- Added a "Scan" button to the frontend for re-reading folders from disk, along with new task invocation endpoints in the API.
- Updated tests across modules to validate the new `scan_directory` task, unique constraints, and database indexing logic.
- Improved cleanup logic for stale files, directories, and thumbnails to handle concurrent operations efficiently.
- Simplified `scan_comics` management command by removing user dependency and aligning it with the new `reconcile` function.
- Enhanced frontend components (`TheComicReader`, `TheComicList`, and others) to include lifecycle method cleanups, prevent memory leaks, and handle task-based scans.

comic/tests/test_models.py

@@ -3,6 +3,7 @@
 from pathlib import Path
 
 import pytest
+from django.db import IntegrityError
 
 from comic import models
 from comic.tests.factories import make_cbz, make_pdf
@@ -69,3 +70,13 @@ def test_classification_comparison() -> None:
         allowed_to_read=models.Directory.Classification.C_PG,
     )
     assert directory.classification > misc.allowed_to_read
+
+
+@pytest.mark.django_db
+def test_duplicate_directory_names_are_rejected() -> None:
+    """The scan depends on this constraint: without it two concurrent requests can create
+    the same directory twice and the browse listing shows it twice."""
+    parent = models.Directory.objects.create(name='Series')
+    models.Directory.objects.create(name='Volume 1', parent=parent)
+    with pytest.raises(IntegrityError):
+        models.Directory.objects.create(name='Volume 1', parent=parent)