Add series fields to audiobooks and update related logic

Introduces 'series' and 'seriesPart' fields to the Audiobook model and database schema. Updates API routes, file organization, and path template utilities to support series metadata. Enhances chapter merging logic, improves notification backend testing, and expands test coverage for admin and API routes.

tests/components/admin/indexers/CategoryTreeView.test.tsx

@@ -0,0 +1,67 @@
+/**
+ * Component: Category Tree View Tests
+ * Documentation: documentation/frontend/components.md
+ */
+
+// @vitest-environment jsdom
+
+import { describe, expect, it, vi } from 'vitest';
+import { fireEvent, render, screen, within } from '@testing-library/react';
+import { CategoryTreeView } from '@/components/admin/indexers/CategoryTreeView';
+import { getChildIds } from '@/lib/utils/torrent-categories';
+
+describe('CategoryTreeView', () => {
+  it('selects parent and children when the parent is toggled', () => {
+    const onChange = vi.fn();
+
+    render(<CategoryTreeView selectedCategories={[]} onChange={onChange} />);
+
+    const audioLabel = screen.getByText('Audio');
+    const audioRow = audioLabel.closest('div')?.parentElement;
+    if (!audioRow) {
+      throw new Error('Audio parent row not found');
+    }
+
+    fireEvent.click(within(audioRow).getByRole('switch'));
+
+    const audioChildren = getChildIds(3000);
+    expect(onChange).toHaveBeenCalledWith(
+      expect.arrayContaining([3000, ...audioChildren])
+    );
+  });
+
+  it('toggles a child category on and off', () => {
+    const onChange = vi.fn();
+
+    render(<CategoryTreeView selectedCategories={[]} onChange={onChange} />);
+
+    const audiobookLabel = screen.getByText('Audiobook');
+    const audiobookRow = audiobookLabel.closest('div')?.parentElement;
+    if (!audiobookRow) {
+      throw new Error('Audiobook row not found');
+    }
+
+    fireEvent.click(within(audiobookRow).getByRole('switch'));
+
+    expect(onChange).toHaveBeenCalledWith(expect.arrayContaining([3030]));
+  });
+
+  it('disables child toggles when all children are selected', () => {
+    const audioChildren = getChildIds(3000);
+
+    render(
+      <CategoryTreeView
+        selectedCategories={[3000, ...audioChildren]}
+        onChange={vi.fn()}
+      />
+    );
+
+    const audiobookLabel = screen.getByText('Audiobook');
+    const audiobookRow = audiobookLabel.closest('div')?.parentElement;
+    if (!audiobookRow) {
+      throw new Error('Audiobook row not found');
+    }
+
+    expect(within(audiobookRow).getByRole('switch')).toBeDisabled();
+  });
+});