**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/migrations/0051_directory_one_directory_name_per_parent.py

@@ -0,0 +1,38 @@
+# Generated by Django 6.1.2 on 2026-10-13 12:51
+
+from collections import defaultdict
+
+from django.db import migrations, models
+
+
+def merge_duplicate_directories(apps, schema_editor) -> None:
+    """Scans could insert the same directory twice, so fold duplicates together
+    before the unique constraint is added, otherwise the migration fails on data
+    the bug has already created."""
+    Directory = apps.get_model('comic', 'Directory')
+    ComicBook = apps.get_model('comic', 'ComicBook')
+
+    groups = defaultdict(list)
+    for directory in Directory.objects.order_by('id'):
+        groups[(directory.parent_id, directory.name)].append(directory)
+
+    for group in groups.values():
+        keeper = group[0]
+        for duplicate in group[1:]:
+            ComicBook.objects.filter(directory=duplicate).update(directory=keeper)
+            duplicate.delete()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('comic', '0050_delete_comicpage'),
+    ]
+
+    operations = [
+        migrations.RunPython(merge_duplicate_directories, migrations.RunPython.noop),
+        migrations.AddConstraint(
+            model_name='directory',
+            constraint=models.UniqueConstraint(fields=('parent', 'name'), name='one_directory_name_per_parent'),
+        ),
+    ]