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/ui/CardSizeControls.test.tsx

@@ -0,0 +1,44 @@
+/**
+ * Component: Card Size Controls Tests
+ * Documentation: documentation/frontend/components.md
+ */
+
+// @vitest-environment jsdom
+
+import React from 'react';
+import { fireEvent, render, screen } from '@testing-library/react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+describe('CardSizeControls', () => {
+  beforeEach(() => {
+    window.innerWidth = 1300;
+  });
+
+  it('moves to the next visible size when zooming in or out', async () => {
+    const onSizeChange = vi.fn();
+    const { CardSizeControls } = await import('@/components/ui/CardSizeControls');
+
+    render(<CardSizeControls size={5} onSizeChange={onSizeChange} />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'Zoom in' }));
+    expect(onSizeChange).toHaveBeenCalledWith(6);
+
+    onSizeChange.mockClear();
+
+    fireEvent.click(screen.getByRole('button', { name: 'Zoom out' }));
+    expect(onSizeChange).toHaveBeenCalledWith(4);
+  });
+
+  it('disables zoom controls at the size boundaries', async () => {
+    const onSizeChange = vi.fn();
+    const { CardSizeControls } = await import('@/components/ui/CardSizeControls');
+
+    const { rerender } = render(<CardSizeControls size={1} onSizeChange={onSizeChange} />);
+
+    expect(screen.getByRole('button', { name: 'Zoom out' })).toBeDisabled();
+
+    rerender(<CardSizeControls size={9} onSizeChange={onSizeChange} />);
+
+    expect(screen.getByRole('button', { name: 'Zoom in' })).toBeDisabled();
+  });
+});