**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.

cbreader/settings/base.py

@@ -37,12 +37,10 @@ INSTALLED_APPS = [
     "django.contrib.staticfiles",
     'drf_yasg',
     'webpack_loader',
-    'bootstrap4',
     "comic",
     'django_extensions',
     'imagekit',
     'django_boost',
-    'sri',
     "corsheaders",
     'django_filters',
     'rest_framework',
@@ -264,6 +262,15 @@ SIMPLE_JWT = {
 # cbreader/urls.py matches the dot separately. See cbreader/urls.py.
 SWAGGER_USE_COMPAT_RENDERERS = False
 
+# Folder scans run through comic.tasks.scan_directory. The immediate backend runs the task
+# in the caller, which is enough today; swapping to a queue backend later changes only
+# this block, not the call sites.
+TASKS = {
+    'default': {
+        'BACKEND': os.getenv('TASK_BACKEND', 'django.tasks.backends.immediate.ImmediateBackend'),
+    },
+}
+
 FRONTEND_DIR = os.path.join(BASE_DIR, 'frontend')
 
 TEMPLATES = [