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/app/api/qbittorrent/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("/qbittorrent/test");
+    return NextResponse.json(data);
+  } catch (error) {
+    const message = error instanceof Error ? error.message : "Failed to test qBittorrent";
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
+}