fix(qbittorrent): stop mislabeling gateway/URL errors as auth failures

The credential tester always reported "Authentication failed — qBittorrent
rejected the credentials" for the qBittorrent service, even when credentials
were correct. test_connection classified any RuntimeError whose message
contained "login failed" as an auth failure — and the gateway-timeout error
(502/503/504 from the reverse proxy) and the wrong-URL diagnostic both started
with "qBittorrent login failed:", so a proxy timeout was reported as a
credentials rejection. That sent users down the wrong path (re-entering correct
passwords to fix a 504).

- QbittorrentClient._login: gateway and URL/routing errors no longer contain
  "login failed"; only a genuine "Fails." body carries the
  "invalid username or password" signal.
- integrations/qbittorrent.test_connection: key the auth message off
  "invalid username or password" specifically; all other login errors flow
  through translate_connection_error so the real reason (proxy timeout, wrong
  URL, empty body) is surfaced.

After this, a failing test reports the actual cause (e.g. "qBittorrent is
unreachable: reverse proxy returned HTTP 504 ...") instead of accusing the
credentials. New regression test asserts a gateway error is NOT reported as
"Authentication failed". 386/386 backend tests pass; ruff clean.

backend/src/media_library_viewer_api/clients/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: backend/src/media_library_viewer_api/clients
 
 ## role
-Provides external service integration clients (API wrappers and command executors) for media servers, identity providers, torrent clients, and remote/local filesystems.
+Provides external service integration clients (API wrappers and remote/local command executors) for media library data aggregation and system monitoring.
 ## parent
 index: backend/src/media_library_viewer_api/.pi-map.index.md
 map: backend/src/media_library_viewer_api/.pi-map.md