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/admin/settings/tabs/EbookTab/EbookTab.tsx

@@ -195,16 +195,6 @@ export function EbookTab({ ebook, onChange, onSuccess, onError, markAsSaved }: E
               </p>
             </div>
           )}
-
-          {/* Coming soon notice */}
-          {ebook.indexerSearchEnabled && (
-            <div className="bg-purple-50 dark:bg-purple-900/20 border border-purple-200 dark:border-purple-800 rounded-lg p-3">
-              <p className="text-sm text-purple-800 dark:text-purple-200">
-                <strong>Coming Soon:</strong> Indexer search for e-books is not yet implemented.
-                Enabling this setting prepares your configuration for when the feature is released.
-              </p>
-            </div>
-          )}
         </div>
       </div>