Add multi-source ebook support and per-indexer categories

Introduces granular toggles for Anna's Archive and Indexer Search as ebook sources, updates settings UI to a three-section layout, and documents the new configuration. Adds per-indexer category configuration with separate tabs for audiobooks and ebooks, updates API routes and types for new settings, and ensures legacy config migration. Indexer grouping and file organization logic now support the new category structure and ebook source toggles.

src/app/admin/page.tsx

@@ -497,7 +497,7 @@ function AdminDashboardContent() {
               </h2>
               <RecentRequestsTable
                 requests={requestsData.requests}
-                ebookSidecarEnabled={settingsData?.ebook?.enabled || false}
+                ebookSidecarEnabled={settingsData?.ebook?.annasArchiveEnabled || settingsData?.ebook?.indexerSearchEnabled || false}
               />
             </div>