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/api/prowlarr/test/route.ts

@@ -0,0 +1,12 @@
+import { NextResponse } from "next/server";
+import { apiFetch } from "@/lib/api";
+
+export async function GET() {
+  try {
+    const data = await apiFetch("/prowlarr/test");
+    return NextResponse.json(data);
+  } catch (error) {
+    const message = error instanceof Error ? error.message : "Failed to test Prowlarr connection";
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
+}