fix(qbittorrent): recognize SID cookie from raw Set-Cookie header on login

qBittorrent (or its reverse proxy) was returning 204 No Content with an SID
cookie and no body on a successful login, but the client only treated the
response as success if body == "Ok." or the cookie was in the parsed
requests cookie jar (resp.cookies.get("SID")). In the reported case the
Set-Cookie header was present (set-cookie=yes) yet the jar was empty —
requests doesn't always populate the jar from such headers (proxy-set/oddly-
attributed cookies) — so a valid login was reported as "Unexpected response".
The user re-entering correct credentials never helped.

Detect a successful login from EITHER the parsed jar OR the raw Set-Cookie
header (cookie name == SID). qBittorrent only sets SID on a valid login, so
this remains authoritative. The diagnostic now lists the cookie names it saw
for future-proofing.

New regression test reproduces the exact 204 + Set-Cookie SID + empty jar
case. 387/387 backend tests pass; ruff clean.

backend/src/media_library_viewer_api/clients/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: backend/src/media_library_viewer_api/clients
 
 ## role
-Collection of API and protocol client wrappers for external services (Jellyfin, Authentik, Jellyseerr, qBittorrent, SSH, local execution) with shared HTTP timeout configuration.
+Provides external service integration clients for authenticating users and aggregating media, filesystem, and torrent data from APIs and remote/local hosts.
 ## parent
 index: backend/src/media_library_viewer_api/.pi-map.index.md
 map: backend/src/media_library_viewer_api/.pi-map.md