Add authors pages and requestType notifications

Introduce full authors browsing/detail feature and enhance notifications to support type-specific titles.

- Add server APIs: authors search, author detail, and author books routes (audnexus integration) that require auth and enrich results with library matches.
- Add frontend pages/components: /authors listing and /authors/[asin] detail pages; AuthorCard, AuthorGrid, AuthorDetailCard, SimilarAuthorsRow, and related skeletons.
- Add hook and integration stubs: new useAuthors hook and audnexus-authors integration; update audible service to expose audibleBaseUrl.
- Update AudiobookDetailsModal to use audibleBaseUrl and link author names to author detail pages.
- Add header navigation link to Authors.
- Notifications: extend docs and code to include requestType (audiobook|ebook), add getEventTitle/getEventMeta helpers, update queue signature and providers/processors/tests to pass/handle requestType so titles can be resolved per request type.
- Misc: job queue, processors, provider tests and notification tests updated to reflect new behavior.

This change enables browsing authors and provides type-aware notification titles without per-provider changes.

tests/processors/send-notification.processor.test.ts

@@ -71,6 +71,33 @@ describe('processSendNotification', () => {
     });
   });
 
+  it('forwards requestType to notification service', async () => {
+    const { processSendNotification } = await import('@/lib/processors/send-notification.processor');
+
+    const payload = {
+      event: 'request_available' as const,
+      requestId: 'req-1',
+      title: 'Test Book',
+      author: 'Test Author',
+      userName: 'Test User',
+      requestType: 'ebook',
+      timestamp: new Date('2024-01-01T00:00:00Z'),
+      jobId: 'job-1',
+    };
+
+    await processSendNotification(payload);
+
+    expect(notificationServiceMock.sendNotification).toHaveBeenCalledWith({
+      event: 'request_available',
+      requestId: 'req-1',
+      title: 'Test Book',
+      author: 'Test Author',
+      userName: 'Test User',
+      requestType: 'ebook',
+      timestamp: expect.any(Date),
+    });
+  });
+
   it('does not throw if notification service fails', async () => {
     notificationServiceMock.sendNotification.mockRejectedValue(new Error('Service error'));