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>

apps/backoffice/lib/api.ts

@@ -910,3 +910,18 @@ export type ProwlarrTestResponse = {
   message: string;
   indexer_count: number | null;
 };
+
+// ---------------------------------------------------------------------------
+// qBittorrent
+// ---------------------------------------------------------------------------
+
+export type QBittorrentAddResponse = {
+  success: boolean;
+  message: string;
+};
+
+export type QBittorrentTestResponse = {
+  success: boolean;
+  message: string;
+  version: string | null;
+};