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/components/layout/Header.tsx

@@ -160,6 +160,12 @@ export function Header() {
             >
               Search
             </Link>
+            <Link
+              href="/authors"
+              className="text-gray-700 dark:text-gray-300 hover:text-blue-600 dark:hover:text-blue-400 transition-colors"
+            >
+              Authors
+            </Link>
             {showBookDate && (
               <Link
                 href="/bookdate"
@@ -264,6 +270,13 @@ export function Header() {
               >
                 Search
               </Link>
+              <Link
+                href="/authors"
+                onClick={() => setShowMobileMenu(false)}
+                className="px-3 py-2 text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-700 rounded-md transition-colors"
+              >
+                Authors
+              </Link>
               {showBookDate && (
                 <Link
                   href="/bookdate"