**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_tasks.py

@@ -0,0 +1,17 @@
+"""Tests for the scan task."""
+
+import pytest
+
+from comic import models, tasks
+from comic.tests.factories import make_cbz
+
+
+@pytest.mark.django_db
+def test_scan_task_runs_through_the_task_backend(comic_volume) -> None:
+    """The immediate backend executes the task in the caller, so this is the whole path."""
+    make_cbz(comic_volume / 'issue.cbz', pages=2)
+
+    tasks.scan_directory.enqueue()
+
+    assert models.ComicBook.objects.count() == 1
+    assert models.ComicBook.objects.get().page_count == 2