feat: add qBittorrent download client integration

Send Prowlarr search results directly to qBittorrent from the modal.
Backend authenticates via SID cookie (login + add torrent endpoints).

- Backend: qbittorrent module with add and test endpoints
- Migration: add qbittorrent settings (url, username, password)
- Settings UI: qBittorrent config card with test connection
- ProwlarrSearchModal: send-to-qBittorrent button per result row
  with spinner/checkmark state progression
- Button only shown when qBittorrent is configured

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

infra/migrations/0044_add_qbittorrent_settings.sql

@@ -0,0 +1,3 @@
+INSERT INTO app_settings (key, value) VALUES
+    ('qbittorrent', '{"url": "", "username": "", "password": ""}')
+ON CONFLICT DO NOTHING;