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/tests/test_credential_tester.py

@@ -76,12 +76,28 @@ class TestQbittorrentTestConnection:
 
     def test_login_failed_translates_to_auth_message(self) -> None:
         mock_client = MagicMock()
-        mock_client.maindata.side_effect = RuntimeError("qBittorrent login failed: Fails.")
+        # Mirrors the real _login auth-failure message for a "Fails." body.
+        mock_client.maindata.side_effect = RuntimeError(
+            "qBittorrent login failed (HTTP 200): invalid username or password"
+        )
         with patch("media_library_viewer_api.integrations.qbittorrent.QbittorrentClient", return_value=mock_client):
             result = qbit_test({"base_url": "http://qb:8080"}, {"username": "u", "password": "p"}, MagicMock())
         assert result.ok is False
         assert "Authentication failed" in result.detail
 
+    def test_gateway_error_does_not_masquerade_as_auth_failure(self) -> None:
+        mock_client = MagicMock()
+        # A 504 from the reverse proxy must NOT be reported as "Authentication
+        # failed" — that misled users into re-entering correct credentials.
+        mock_client.maindata.side_effect = RuntimeError(
+            "qBittorrent is unreachable: reverse proxy returned HTTP 504 for http://qb:8080/api/v2/auth/login."
+        )
+        with patch("media_library_viewer_api.integrations.qbittorrent.QbittorrentClient", return_value=mock_client):
+            result = qbit_test({"base_url": "http://qb:8080"}, {"username": "u", "password": "p"}, MagicMock())
+        assert result.ok is False
+        assert "Authentication failed" not in result.detail
+        assert "504" in result.detail
+
     def test_connection_error_translates(self) -> None:
         mock_client = MagicMock()
         mock_client.maindata.side_effect = requests.ConnectionError("Connection refused")