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/api/src/main.rs

@@ -13,6 +13,7 @@ mod api_middleware;
 mod openapi;
 mod pages;
 mod prowlarr;
+mod qbittorrent;
 mod reading_progress;
 mod search;
 mod settings;
@@ -107,6 +108,8 @@ async fn main() -> anyhow::Result<()> {
         .route("/admin/tokens/:id/delete", axum::routing::post(tokens::delete_token))
         .route("/prowlarr/search", axum::routing::post(prowlarr::search_prowlarr))
         .route("/prowlarr/test", get(prowlarr::test_prowlarr))
+        .route("/qbittorrent/add", axum::routing::post(qbittorrent::add_torrent))
+        .route("/qbittorrent/test", get(qbittorrent::test_qbittorrent))
         .route("/komga/sync", axum::routing::post(komga::sync_komga_read_books))
         .route("/komga/reports", get(komga::list_sync_reports))
         .route("/komga/reports/:id", get(komga::get_sync_report))