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/app/setup/steps/BackendSelectionStep.test.tsx

@@ -0,0 +1,73 @@
+/**
+ * Component: Backend Selection Step Tests
+ * Documentation: documentation/setup-wizard.md
+ */
+
+// @vitest-environment jsdom
+
+import React from 'react';
+import { fireEvent, render, screen } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+
+describe('BackendSelectionStep', () => {
+  it('updates the audible region helper based on backend', async () => {
+    const { BackendSelectionStep } = await import('@/app/setup/steps/BackendSelectionStep');
+
+    const { rerender } = render(
+      <BackendSelectionStep
+        value="plex"
+        onChange={vi.fn()}
+        audibleRegion="us"
+        onAudibleRegionChange={vi.fn()}
+        onNext={vi.fn()}
+        onBack={vi.fn()}
+      />
+    );
+
+    expect(screen.getByText(/configuration in Plex/i)).toBeInTheDocument();
+
+    rerender(
+      <BackendSelectionStep
+        value="audiobookshelf"
+        onChange={vi.fn()}
+        audibleRegion="us"
+        onAudibleRegionChange={vi.fn()}
+        onNext={vi.fn()}
+        onBack={vi.fn()}
+      />
+    );
+
+    expect(screen.getByText(/configuration in Audiobookshelf/i)).toBeInTheDocument();
+  });
+
+  it('updates backend selection and audible region', async () => {
+    const onChange = vi.fn();
+    const onAudibleRegionChange = vi.fn();
+    const onNext = vi.fn();
+    const onBack = vi.fn();
+    const { BackendSelectionStep } = await import('@/app/setup/steps/BackendSelectionStep');
+
+    render(
+      <BackendSelectionStep
+        value="plex"
+        onChange={onChange}
+        audibleRegion="us"
+        onAudibleRegionChange={onAudibleRegionChange}
+        onNext={onNext}
+        onBack={onBack}
+      />
+    );
+
+    fireEvent.click(screen.getByRole('radio', { name: /Audiobookshelf/i }));
+    expect(onChange).toHaveBeenCalledWith('audiobookshelf');
+
+    fireEvent.change(screen.getByLabelText('Audible Region'), { target: { value: 'uk' } });
+    expect(onAudibleRegionChange).toHaveBeenCalledWith('uk');
+
+    fireEvent.click(screen.getByRole('button', { name: 'Back' }));
+    fireEvent.click(screen.getByRole('button', { name: 'Next' }));
+
+    expect(onBack).toHaveBeenCalled();
+    expect(onNext).toHaveBeenCalled();
+  });
+});