fix(qbittorrent): show set-cookie presence in login diagnostic

When the login endpoint returns an unexpected response (e.g. a 204 No Content
with no body), the diagnostic now reports whether a Set-Cookie header was
present. That single fact tells us whether qBittorrent attempted to establish
a session at all — distinguishing "qBittorrent answered weirdly" from
"something in the proxy path answered before qBittorrent" (e.g. a 204 from a
misrouted reverse proxy), which is the key clue when diagnosing login failures
behind a proxy.

42/42 qBittorrent + credential-tester tests pass; ruff clean.

backend/src/media_library_viewer_api/clients/qbittorrent.py

@@ -84,11 +84,12 @@ class QbittorrentClient:
             return
         if body == "Fails.":
             raise RuntimeError(f"qBittorrent login failed (HTTP {resp.status_code}): invalid username or password")
+        set_cookie = "yes" if resp.headers.get("Set-Cookie") else "no"
         raise RuntimeError(
             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)."
+            f"body={body!r}, set-cookie={set_cookie}). Expected the text 'Ok.' (or an SID cookie) "
+            "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)."
         )
 
     def _get(self, path: str, **params: Any) -> dict[str, Any]: