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/integrations/qbittorrent.py

@@ -42,8 +42,10 @@ def test_connection(
         return TestResult(ok=True, detail="Connected to qBittorrent.", evidence=version)
     except RuntimeError as exc:
         lowered = str(exc).lower()
-        if "login failed" in lowered:
+        if "invalid username or password" in lowered:
             return TestResult(ok=False, detail="Authentication failed — qBittorrent rejected the credentials.")
+        # Gateway timeout, wrong URL/path, empty body, etc. — surface the real
+        # reason instead of masking every login error as an auth failure.
         return translate_connection_error(exc, context="qBittorrent")
     except Exception as exc:
         return translate_connection_error(exc, context="qBittorrent")