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/api/admin-settings-core.routes.test.ts

@@ -129,6 +129,100 @@ describe('Admin settings core routes', () => {
     expect(invalidateQbMock).toHaveBeenCalled();
   });
 
+  it('rejects invalid download client types', async () => {
+    const request = {
+      json: vi.fn().mockResolvedValue({
+        type: 'transmission',
+        url: 'http://transmission',
+      }),
+    };
+
+    const { PUT } = await import('@/app/api/admin/settings/download-client/route');
+    const response = await PUT(request as any);
+    const payload = await response.json();
+
+    expect(response.status).toBe(400);
+    expect(payload.error).toMatch(/Invalid client type/);
+  });
+
+  it('rejects missing qBittorrent credentials', async () => {
+    const request = {
+      json: vi.fn().mockResolvedValue({
+        type: 'qbittorrent',
+        url: 'http://qbt',
+        password: 'pass',
+        remotePathMappingEnabled: false,
+      }),
+    };
+
+    const { PUT } = await import('@/app/api/admin/settings/download-client/route');
+    const response = await PUT(request as any);
+    const payload = await response.json();
+
+    expect(response.status).toBe(400);
+    expect(payload.error).toMatch(/URL, username, and password/);
+  });
+
+  it('rejects missing SABnzbd credentials', async () => {
+    const request = {
+      json: vi.fn().mockResolvedValue({
+        type: 'sabnzbd',
+        url: 'http://sab',
+        remotePathMappingEnabled: false,
+      }),
+    };
+
+    const { PUT } = await import('@/app/api/admin/settings/download-client/route');
+    const response = await PUT(request as any);
+    const payload = await response.json();
+
+    expect(response.status).toBe(400);
+    expect(payload.error).toMatch(/API key/);
+  });
+
+  it('rejects path mapping when required fields are missing', async () => {
+    const request = {
+      json: vi.fn().mockResolvedValue({
+        type: 'qbittorrent',
+        url: 'http://qbt',
+        username: 'user',
+        password: 'pass',
+        remotePathMappingEnabled: true,
+      }),
+    };
+
+    const { PUT } = await import('@/app/api/admin/settings/download-client/route');
+    const response = await PUT(request as any);
+    const payload = await response.json();
+
+    expect(response.status).toBe(400);
+    expect(payload.error).toMatch(/Remote path and local path/);
+  });
+
+  it('rejects invalid path mapping configuration', async () => {
+    pathMapperMock.validate.mockImplementationOnce(() => {
+      throw new Error('bad mapping');
+    });
+    const request = {
+      json: vi.fn().mockResolvedValue({
+        type: 'qbittorrent',
+        url: 'http://qbt',
+        username: 'user',
+        password: 'pass',
+        remotePathMappingEnabled: true,
+        remotePath: '/remote',
+        localPath: '/local',
+      }),
+    };
+
+    const { PUT } = await import('@/app/api/admin/settings/download-client/route');
+    const response = await PUT(request as any);
+    const payload = await response.json();
+
+    expect(response.status).toBe(400);
+    expect(payload.error).toMatch(/bad mapping/);
+  });
+
   it('updates paths settings', async () => {
     const request = {
       json: vi.fn().mockResolvedValue({