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/qbittorrent.py

@@ -68,7 +68,7 @@ class QbittorrentClient:
         # than as a bare HTTPError.
         if resp.status_code in (502, 503, 504):
             raise RuntimeError(
-                f"qBittorrent login failed: reverse proxy returned HTTP {resp.status_code} "
+                f"qBittorrent is unreachable: reverse proxy returned HTTP {resp.status_code} "
                 f"for {resp.url}. qBittorrent may be down, starting up, or unable to "
                 "answer within the proxy's forwarding timeout."
             )
@@ -85,9 +85,9 @@ class QbittorrentClient:
         if body == "Fails.":
             raise RuntimeError(f"qBittorrent login failed (HTTP {resp.status_code}): invalid username or password")
         raise RuntimeError(
-            f"qBittorrent login failed (HTTP {resp.status_code}, body={body!r}). "
-            "Expected the text 'Ok.' from /api/v2/auth/login — this usually means "
-            "base_url does not reach the qBittorrent Web API (check the URL, path, "
+            f"Unexpected response from qBittorrent login endpoint (HTTP {resp.status_code}, "
+            f"body={body!r}). Expected the text 'Ok.' from /api/v2/auth/login — this usually "
+            "means base_url does not reach the qBittorrent Web API (check the URL, path, "
             "and any reverse proxy in front of qBittorrent)."
         )