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>

infra/migrations/0043_add_prowlarr_settings.sql

@@ -0,0 +1,3 @@
+INSERT INTO app_settings (key, value) VALUES
+    ('prowlarr', '{"url": "", "api_key": "", "categories": [7030, 7020]}')
+ON CONFLICT DO NOTHING;