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/processors/plex-recently-added.processor.ts

@@ -325,7 +325,9 @@ export async function processPlexRecentlyAddedCheck(payload: PlexRecentlyAddedPa
               request.id,
               audiobook.title,
               audiobook.author,
-              request.user.plexUsername || 'Unknown User'
+              request.user.plexUsername || 'Unknown User',
+              undefined,
+              'audiobook'
             ).catch((error) => {
               logger.error('Failed to queue notification', { error: error instanceof Error ? error.message : String(error) });
             });