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.

src/lib/hooks/useAudiobooks.ts

@@ -12,6 +12,7 @@ export interface Audiobook {
   asin: string;
   title: string;
   author: string;
+  authorAsin?: string;
   narrator?: string;
   description?: string;
   coverArtUrl?: string;
@@ -81,6 +82,7 @@ export function useAudiobookDetails(asin: string | null) {
 
   return {
     audiobook: data?.audiobook || null,
+    audibleBaseUrl: data?.audibleBaseUrl || 'https://www.audible.com',
     isLoading,
     error,
   };