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/PathsStep.test.tsx

@@ -0,0 +1,100 @@
+/**
+ * Component: Setup Paths Step Tests
+ * Documentation: documentation/setup-wizard.md
+ */
+
+// @vitest-environment jsdom
+
+import React, { useState } from 'react';
+import { fireEvent, render, screen, waitFor } from '@testing-library/react';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { PathsStep } from '@/app/setup/steps/PathsStep';
+
+const PathsHarness = ({
+  onNext,
+  onBack,
+  initialState,
+}: {
+  onNext: () => void;
+  onBack: () => void;
+  initialState?: Partial<React.ComponentProps<typeof PathsStep>>;
+}) => {
+  const [state, setState] = useState({
+    downloadDir: '/downloads',
+    mediaDir: '/media/audiobooks',
+    metadataTaggingEnabled: true,
+    chapterMergingEnabled: false,
+    ...initialState,
+  });
+
+  return (
+    <PathsStep
+      {...state}
+      onUpdate={(field, value) => setState((prev) => ({ ...prev, [field]: value }))}
+      onNext={onNext}
+      onBack={onBack}
+    />
+  );
+};
+
+describe('PathsStep', () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('validates paths and allows navigation on success', async () => {
+    const fetchMock = vi.fn().mockResolvedValue({
+      ok: true,
+      json: async () => ({
+        success: true,
+        message: 'Directories are ready',
+        downloadDirValid: true,
+        mediaDirValid: true,
+      }),
+    });
+    vi.stubGlobal('fetch', fetchMock);
+    const onNext = vi.fn();
+
+    render(<PathsHarness onNext={onNext} onBack={vi.fn()} />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'Validate Paths' }));
+
+    await waitFor(() => {
+      expect(fetchMock).toHaveBeenCalledWith('/api/setup/test-paths', expect.any(Object));
+    });
+
+    expect(screen.getByText('Directories are ready')).toBeInTheDocument();
+
+    fireEvent.click(screen.getByRole('button', { name: 'Next' }));
+    expect(onNext).toHaveBeenCalled();
+  });
+
+  it('requires validation before proceeding', async () => {
+    const onNext = vi.fn();
+    render(<PathsHarness onNext={onNext} onBack={vi.fn()} />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'Next' }));
+
+    expect(screen.getByText('Please validate the paths before proceeding')).toBeInTheDocument();
+    expect(onNext).not.toHaveBeenCalled();
+  });
+
+  it('toggles metadata and chapter merge settings', async () => {
+    render(
+      <PathsHarness
+        onNext={vi.fn()}
+        onBack={vi.fn()}
+        initialState={{ metadataTaggingEnabled: false, chapterMergingEnabled: false }}
+      />
+    );
+
+    const metadataToggle = screen.getByLabelText('Auto-tag audio files with metadata');
+    const chapterToggle = screen.getByLabelText('Auto-merge chapters to M4B');
+
+    fireEvent.click(metadataToggle);
+    fireEvent.click(chapterToggle);
+
+    expect(metadataToggle).toBeChecked();
+    expect(chapterToggle).toBeChecked();
+  });
+});