Add multi-source ebook search & processing

Refactor ebook flow to support multiple sources (Anna's Archive direct downloads + Prowlarr indexer search) and unify handling with existing audiobook processors. Key changes:
- search-ebook.processor: rewritten to try Anna's Archive first then fall back to indexer search, add Prowlarr grouping, ranking (rankEbookTorrents), and handlers to route results to direct-download or download-torrent flows.
- organize-files.processor: enriches audiobook/ebook metadata from AudibleCache (year, narrator), treats indexer downloads specially (seed retention), adds optional NZB cleanup/archive logic, and improves retryable error detection.
- file-organizer: organizeEbook now accepts additional metadata and an isIndexerDownload flag and supports directories vs single-file paths.
- API/UI: include request.type in admin requests API and remove the “coming soon” notice from Ebook settings tab.
- fetch-ebook route: removed blocking error for indexer-only mode so the flow can proceed when indexer search is enabled.
- Documentation: update TOC, ebook-sidecar, settings-pages, and ranking-algorithm docs to describe indexer search, unified ebook ranking, configuration, and flows.
These changes enable indexer-based ebook discovery, ranking, and downloads while preserving existing Anna's Archive behavior and reusing audiobook download processors where possible.

src/app/api/admin/requests/route.ts

@@ -130,6 +130,7 @@ export async function GET(request: NextRequest) {
           title: request.audiobook.title,
           author: request.audiobook.author,
           status: request.status,
+          type: request.type || 'audiobook', // Include request type for UI display
           userId: request.user.id,
           user: request.user.plexUsername,
           createdAt: request.createdAt,