feat: add Prowlarr integration for manual release search

Add Prowlarr indexer integration (step 1: config + manual search).
Allows searching for comics/ebooks releases on Prowlarr indexers
directly from the series detail page, with download links and
per-volume search for missing books.

- Backend: new prowlarr module with search and test endpoints
- Migration: add prowlarr settings (url, api_key, categories)
- Settings UI: Prowlarr config card with test connection button
- ProwlarrSearchModal: auto-search on open, missing volumes shortcuts
- Fix series.readCount i18n plural parameter on series pages

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/app/libraries/[id]/series/page.tsx

@@ -95,7 +95,7 @@ export default async function LibrarySeriesPage({
                     </h3>
                     <div className="flex items-center justify-between mt-1">
                       <p className="text-xs text-muted-foreground">
-                        {t("series.readCount", { read: String(s.books_read_count), total: String(s.book_count) })}
+                        {t("series.readCount", { read: String(s.books_read_count), total: String(s.book_count), plural: s.book_count !== 1 ? "s" : "" })}
                       </p>
                       <MarkSeriesReadButton
                         seriesName={s.name}