fix(qbittorrent): reuse authenticated client across widget fetches

QbittorrentWidgetSource built a brand-new QbittorrentClient on every fetch,
logging in each time. With three qBittorrent widgets polling every 5-30s and
qBittorrent verifying passwords with slow PBKDF2 hashing, the concurrent login
load saturates its web thread pool and the reverse proxy returns 504 gateway
timeouts on /api/v2/auth/login. The client was already designed for reuse
(login once, SID cookie reuse, 403 re-login) — the source just wasn't using it.

Cache one QbittorrentClient per service (lru_cache keyed by service id, URL,
credentials, timeout) so the SID cookie persists across fetches and login
happens once. Mirrors dependencies._jellyfin_client_for. A credentials/URL
change produces a new cache key, so stale clients aren't reused after
reconfiguration.

Also surface 502/503/504 from the login as a clear "reverse proxy returned
HTTP <code> ... qBittorrent may be down/starting/overloaded" RuntimeError
instead of a bare HTTPError, so future gateway issues read as infrastructure,
not auth.

Tests: autouse fixture clears the client cache between tests; new gateway-error
login test. 385/385 backend tests pass; ruff clean.

backend/src/media_library_viewer_api/clients/qbittorrent.py

@@ -61,6 +61,17 @@ class QbittorrentClient:
             timeout=self.timeout,
             headers={"Referer": self.base_url},
         )
+        # 502/503/504 come from the reverse proxy when qBittorrent is down,
+        # starting up, or can't answer within the proxy's forwarding timeout
+        # (qBittorrent's PBKDF2 password check is intentionally slow, so a
+        # flood of concurrent logins can trip this). Surface it clearly rather
+        # 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"for {resp.url}. qBittorrent may be down, starting up, or unable to "
+                "answer within the proxy's forwarding timeout."
+            )
         resp.raise_for_status()
         body = resp.text.strip()
         # Some reverse proxies forward the SID cookie but mangle the text body;