**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/tasks.py

@@ -0,0 +1,18 @@
+"""The folder scan as a task.
+
+The default backend runs tasks immediately, so this behaves like a function call today.
+Writing it as a task is what lets the scan move to a worker later without changing any
+of the call sites in comic/rest.py or the management command.
+"""
+
+from django.tasks import task
+
+from comic import models
+from comic.processing import reconcile
+
+
+@task
+def scan_directory(selector: str | None = None) -> None:
+    """Reconcile one directory, or the root of the comic volume when no selector is given."""
+    directory = models.Directory.objects.filter(selector=selector).first() if selector else None
+    reconcile(directory)